diff --git a/cmd/guru/emacs-test.bash b/cmd/guru/emacs-test.bash
index 8dcc8b7556..5036d3d0d5 100755
--- a/cmd/guru/emacs-test.bash
+++ b/cmd/guru/emacs-test.bash
@@ -35,6 +35,7 @@ emacs --batch --no-splash --no-window-system --no-init \
 (progn
   (princ (emacs-version)) ; requires Emacs v23
   (find-file "'$thisdir'/main.go")
+  (insert "// modify but do not save the editor buffer\n")
   (search-forward "\"fmt\"")
   (backward-char)
   (go-guru-describe)
diff --git a/cmd/guru/guru.el b/cmd/guru/guru.el
index d00f44c69a..e9fed47199 100644
--- a/cmd/guru/guru.el
+++ b/cmd/guru/guru.el
@@ -86,11 +86,6 @@ a scope if not already set.  Process the output to replace each
 file name with a small hyperlink.  Display the result."
   (if (not buffer-file-name)
       (error "Cannot use guru on a buffer without a file name"))
-  ;; It's not sufficient to save a modified buffer since if
-  ;; gofmt-before-save is on the before-save-hook, saving will
-  ;; disturb the selected region.
-  (if (buffer-modified-p)
-      (error "Please save the buffer before invoking go-guru"))
   (and need-scope
        (string-equal "" go-guru-scope)
        (go-guru-set-scope))
@@ -105,21 +100,34 @@ file name with a small hyperlink.  Display the result."
 			 (1- (position-bytes (point))))))
          (env-vars (go-root-and-paths))
          (goroot-env (concat "GOROOT=" (car env-vars)))
-         (gopath-env (concat "GOPATH=" (mapconcat #'identity (cdr env-vars) ":"))))
-    (with-current-buffer (get-buffer-create "*go-guru*")
+         (gopath-env (concat "GOPATH=" (mapconcat #'identity (cdr env-vars) ":")))
+	 (output-buffer (get-buffer-create "*go-guru*")))
+    (with-current-buffer output-buffer
       (setq buffer-read-only nil)
       (erase-buffer)
-      (insert "Go Guru\n")
-      (let ((args (list go-guru-command nil t nil
-			"-scope" go-guru-scope mode posn)))
-        ;; Log the command to *Messages*, for debugging.
-        (message "Command: %s:" args)
-        (message nil) ; clears/shrinks minibuffer
-
-        (message "Running guru...")
-        ;; Use dynamic binding to modify/restore the environment
-        (let ((process-environment (list* goroot-env gopath-env process-environment)))
-            (apply #'call-process args)))
+      (insert "Go Guru\n"))
+    (with-current-buffer (get-buffer-create "*go-guru-input*")
+      (setq buffer-read-only nil)
+      (erase-buffer)
+      (go-guru--insert-modified-files)
+      (let* ((args (list "-modified"
+			 "-scope" go-guru-scope
+			 mode
+			 posn)))
+	;; Log the command to *Messages*, for debugging.
+	(message "Command: %s:" args)
+	(message nil) ; clears/shrinks minibuffer
+	(message "Running guru...")
+	;; Use dynamic binding to modify/restore the environment
+	(let ((process-environment (list* goroot-env gopath-env process-environment)))
+	  (apply #'call-process-region (append (list (point-min)
+						     (point-max)
+						     go-guru-command
+						     nil ; delete
+						     output-buffer
+						     t)
+					       args)))))
+    (with-current-buffer output-buffer
       (insert "\n")
       (compilation-mode)
       (setq compilation-error-screen-columns nil)
@@ -158,6 +166,20 @@ file name with a small hyperlink.  Display the result."
         (shrink-window-if-larger-than-buffer w)
         (set-window-point w (point-min))))))
 
+(defun go-guru--insert-modified-files ()
+  "Insert the contents of each modified Go buffer into the
+current buffer in the format specified by guru's -modified flag."
+  (mapc #'(lambda (b)
+	    (and (buffer-modified-p b)
+		 (buffer-file-name b)
+		 (string= (file-name-extension (buffer-file-name b)) "go")
+		 (progn
+		   (insert (format "%s\n%d\n"
+				   (buffer-file-name b)
+				   (buffer-size b)))
+		   (insert-buffer-substring b))))
+	(buffer-list)))
+
 (defun go-guru-callees ()
   "Show possible callees of the function call at the current point."
   (interactive)
diff --git a/cmd/guru/guru.go b/cmd/guru/guru.go
index 8d2da63d54..d48f308892 100644
--- a/cmd/guru/guru.go
+++ b/cmd/guru/guru.go
@@ -252,7 +252,21 @@ func parseQueryPos(lprog *loader.Program, pos string, needExact bool) (*queryPos
 	if err != nil {
 		return nil, err
 	}
-	start, end, err := findQueryPos(lprog.Fset, filename, startOffset, endOffset)
+
+	// Find the named file among those in the loaded program.
+	var file *token.File
+	lprog.Fset.Iterate(func(f *token.File) bool {
+		if sameFile(filename, f.Name()) {
+			file = f
+			return false // done
+		}
+		return true // continue
+	})
+	if file == nil {
+		return nil, fmt.Errorf("file %s not found in loaded program", filename)
+	}
+
+	start, end, err := fileOffsetToPos(file, startOffset, endOffset)
 	if err != nil {
 		return nil, err
 	}
diff --git a/cmd/guru/main.go b/cmd/guru/main.go
index 0b85f03e9c..538bcd5bbc 100644
--- a/cmd/guru/main.go
+++ b/cmd/guru/main.go
@@ -13,15 +13,19 @@ package main // import "golang.org/x/tools/cmd/guru"
 
 import (
 	"bufio"
+	"bytes"
 	"encoding/json"
 	"encoding/xml"
 	"flag"
 	"fmt"
 	"go/build"
 	"io"
+	"io/ioutil"
 	"log"
 	"os"
+	"path/filepath"
 	"runtime/pprof"
+	"strconv"
 	"strings"
 
 	"golang.org/x/tools/go/buildutil"
@@ -29,6 +33,7 @@ import (
 
 // flags
 var (
+	modifiedFlag   = flag.Bool("modified", false, "read archive of modified files from standard input")
 	scopeFlag      = flag.String("scope", "", "comma-separated list of `packages` the analysis should be limited to (default=all)")
 	ptalogFlag     = flag.String("ptalog", "", "write points-to analysis log to `file`")
 	formatFlag     = flag.String("format", "plain", "output `format`; one of {plain,json,xml}")
@@ -72,6 +77,14 @@ The -format flag controls the output format:
 	json	structured data in JSON syntax.
 	xml	structured data in XML syntax.
 
+The -modified flag causes guru to read an archive from standard input.
+
+	Files in this archive will be used in preference to those in
+	the file system.  In this way, a text editor may supply guru
+	with the contents of its unsaved buffers.  Each archive entry
+	consists of the file name, a newline, the decimal file size,
+	another newline, and the contents of the file.
+
 User manual: http://golang.org/s/oracle-user-manual
 
 Example: describe syntax at offset 530 in this file (an import spec):
@@ -150,11 +163,31 @@ func main() {
 		log.Fatalf("illegal -format value: %q.\n"+useHelp, *formatFlag)
 	}
 
+	ctxt := &build.Default
+
+	// If there were modified files,
+	// read them from the standard input and
+	// overlay them on the build context.
+	if *modifiedFlag {
+		modified, err := parseArchive(os.Stdin)
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		// All I/O done by guru needs to consult the modified map.
+		// The ReadFile done by referrers does,
+		// but the loader's cgo preprocessing currently does not.
+
+		if len(modified) > 0 {
+			ctxt = useModifiedFiles(ctxt, modified)
+		}
+	}
+
 	// Ask the guru.
 	query := Query{
 		Mode:       mode,
 		Pos:        posn,
-		Build:      &build.Default,
+		Build:      ctxt,
 		Scope:      strings.Split(*scopeFlag, ","),
 		PTALog:     ptalog,
 		Reflection: *reflectFlag,
@@ -184,3 +217,68 @@ func main() {
 		query.WriteTo(os.Stdout)
 	}
 }
+
+func parseArchive(archive io.Reader) (map[string][]byte, error) {
+	modified := make(map[string][]byte)
+	r := bufio.NewReader(archive)
+	for {
+		// Read file name.
+		filename, err := r.ReadString('\n')
+		if err != nil {
+			if err == io.EOF {
+				break // OK
+			}
+			return nil, fmt.Errorf("reading modified file name: %v", err)
+		}
+		filename = filepath.Clean(strings.TrimSpace(filename))
+
+		// Read file size.
+		sz, err := r.ReadString('\n')
+		if err != nil {
+			return nil, fmt.Errorf("reading size of modified file %s: %v", filename, err)
+		}
+		sz = strings.TrimSpace(sz)
+		size, err := strconv.ParseInt(sz, 10, 32)
+		if err != nil {
+			return nil, fmt.Errorf("parsing size of modified file %s: %v", filename, err)
+		}
+
+		// Read file content.
+		var content bytes.Buffer
+		content.Grow(int(size))
+		if _, err := io.CopyN(&content, r, size); err != nil {
+			return nil, fmt.Errorf("reading modified file %s: %v", filename, err)
+		}
+		modified[filename] = content.Bytes()
+	}
+
+	return modified, nil
+}
+
+// useModifiedFiles augments the provided build.Context by the
+// mapping from file names to alternative contents.
+func useModifiedFiles(orig *build.Context, modified map[string][]byte) *build.Context {
+	rc := func(data []byte) (io.ReadCloser, error) {
+		return ioutil.NopCloser(bytes.NewBuffer(data)), nil
+	}
+
+	copy := *orig // make a copy
+	ctxt := &copy
+	ctxt.OpenFile = func(path string) (io.ReadCloser, error) {
+		// Fast path: names match exactly.
+		if content, ok := modified[path]; ok {
+			return rc(content)
+		}
+
+		// Slow path: check for same file under a different
+		// alias, perhaps due to a symbolic link.
+		for filename, content := range modified {
+			if sameFile(path, filename) {
+				return rc(content)
+			}
+		}
+
+		return buildutil.OpenFile(orig, path)
+	}
+	return ctxt
+}
diff --git a/cmd/guru/pos.go b/cmd/guru/pos.go
index d9235b46c2..4566c06b4a 100644
--- a/cmd/guru/pos.go
+++ b/cmd/guru/pos.go
@@ -66,26 +66,11 @@ func parsePos(pos string) (filename string, startOffset, endOffset int, err erro
 	return
 }
 
-// findQueryPos searches fset for filename and translates the
-// specified file-relative byte offsets into token.Pos form.  It
-// returns an error if the file was not found or the offsets were out
-// of bounds.
+// fileOffsetToPos translates the specified file-relative byte offsets
+// into token.Pos form.  It returns an error if the file was not found
+// or the offsets were out of bounds.
 //
-func findQueryPos(fset *token.FileSet, filename string, startOffset, endOffset int) (start, end token.Pos, err error) {
-	var file *token.File
-	fset.Iterate(func(f *token.File) bool {
-		if sameFile(filename, f.Name()) {
-			// (f.Name() is absolute)
-			file = f
-			return false // done
-		}
-		return true // continue
-	})
-	if file == nil {
-		err = fmt.Errorf("couldn't find file containing position")
-		return
-	}
-
+func fileOffsetToPos(file *token.File, startOffset, endOffset int) (start, end token.Pos, err error) {
 	// Range check [start..end], inclusive of both end-points.
 
 	if 0 <= startOffset && startOffset <= file.Size() {
@@ -119,8 +104,8 @@ func sameFile(x, y string) bool {
 	return false
 }
 
-// fastQueryPos parses the position string and returns a QueryPos.
-// It parses only a single file, and does not run the type checker.
+// fastQueryPos parses the position string and returns a queryPos.
+// It parses only a single file and does not run the type checker.
 func fastQueryPos(pos string) (*queryPos, error) {
 	filename, startOffset, endOffset, err := parsePos(pos)
 	if err != nil {
@@ -133,7 +118,7 @@ func fastQueryPos(pos string) (*queryPos, error) {
 		return nil, err
 	}
 
-	start, end, err := findQueryPos(fset, filename, startOffset, endOffset)
+	start, end, err := fileOffsetToPos(fset.File(f.Pos()), startOffset, endOffset)
 	if err != nil {
 		return nil, err
 	}
diff --git a/cmd/guru/referrers.go b/cmd/guru/referrers.go
index 53d60273a8..aae82dbea5 100644
--- a/cmd/guru/referrers.go
+++ b/cmd/guru/referrers.go
@@ -8,12 +8,14 @@ import (
 	"bytes"
 	"fmt"
 	"go/ast"
+	"go/build"
 	"go/token"
 	"go/types"
-	"io/ioutil"
+	"io"
 	"sort"
 
 	"golang.org/x/tools/cmd/guru/serial"
+	"golang.org/x/tools/go/buildutil"
 	"golang.org/x/tools/go/loader"
 	"golang.org/x/tools/refactor/importgraph"
 )
@@ -108,6 +110,7 @@ func referrers(q *Query) error {
 	sort.Sort(byNamePos{q.Fset, refs})
 
 	q.result = &referrersResult{
+		build: q.Build,
 		qpos:  qpos,
 		query: id,
 		obj:   obj,
@@ -156,6 +159,7 @@ func (p byNamePos) Less(i, j int) bool {
 }
 
 type referrersResult struct {
+	build *build.Context
 	qpos  *queryPos
 	query *ast.Ident   // identifier of query
 	obj   types.Object // object it denotes
@@ -188,7 +192,7 @@ func (r *referrersResult) display(printf printfFunc) {
 			// start asynchronous read.
 			go func() {
 				sema <- struct{}{} // acquire token
-				content, err := ioutil.ReadFile(posn.Filename)
+				content, err := readFile(r.build, posn.Filename)
 				<-sema // release token
 				if err != nil {
 					fi.data <- err
@@ -224,6 +228,21 @@ func (r *referrersResult) display(printf printfFunc) {
 	}
 }
 
+// readFile is like ioutil.ReadFile, but
+// it goes through the virtualized build.Context.
+func readFile(ctxt *build.Context, filename string) ([]byte, error) {
+	rc, err := buildutil.OpenFile(ctxt, filename)
+	if err != nil {
+		return nil, err
+	}
+	defer rc.Close()
+	var buf bytes.Buffer
+	if _, err := io.Copy(&buf, rc); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
 // TODO(adonovan): encode extent, not just Pos info, in Serial form.
 
 func (r *referrersResult) toSerial(res *serial.Result, fset *token.FileSet) {