diff --git a/src/pkg/path/filepath/path.go b/src/pkg/path/filepath/path.go
index c40d9ff5644..afb8f102d57 100644
--- a/src/pkg/path/filepath/path.go
+++ b/src/pkg/path/filepath/path.go
@@ -258,6 +258,75 @@ func Abs(path string) (string, os.Error) {
 	return Join(wd, path), nil
 }
 
+// Rel returns a relative path that is lexically equivalent to targpath when
+// joined to basepath with an intervening separator. That is,
+// Join(basepath, Rel(basepath, targpath)) is equivalent to targpath itself.
+// An error is returned if targpath can't be made relative to basepath or if
+// knowing the current working directory would be necessary to compute it.
+func Rel(basepath, targpath string) (string, os.Error) {
+	baseVol := VolumeName(basepath)
+	targVol := VolumeName(targpath)
+	base := Clean(basepath)
+	targ := Clean(targpath)
+	if targ == base {
+		return ".", nil
+	}
+	base = base[len(baseVol):]
+	targ = targ[len(targVol):]
+	if base == "." {
+		base = ""
+	}
+	// Can't use IsAbs - `\a` and `a` are both relative in Windows.
+	baseSlashed := len(base) > 0 && base[0] == Separator
+	targSlashed := len(targ) > 0 && targ[0] == Separator
+	if baseSlashed != targSlashed || baseVol != targVol {
+		return "", os.NewError("Rel: can't make " + targ + " relative to " + base)
+	}
+	// Position base[b0:bi] and targ[t0:ti] at the first differing elements.
+	bl := len(base)
+	tl := len(targ)
+	var b0, bi, t0, ti int
+	for {
+		for bi < bl && base[bi] != Separator {
+			bi++
+		}
+		for ti < tl && targ[ti] != Separator {
+			ti++
+		}
+		if targ[t0:ti] != base[b0:bi] {
+			break
+		}
+		if bi < bl {
+			bi++
+		}
+		if ti < tl {
+			ti++
+		}
+		b0 = bi
+		t0 = ti
+	}
+	if base[b0:bi] == ".." {
+		return "", os.NewError("Rel: can't make " + targ + " relative to " + base)
+	}
+	if b0 != bl {
+		// Base elements left. Must go up before going down.
+		seps := strings.Count(base[b0:bl], string(Separator))
+		buf := make([]byte, 3+seps*3+tl-t0)
+		n := copy(buf, "..")
+		for i := 0; i < seps; i++ {
+			buf[n] = Separator
+			copy(buf[n+1:], "..")
+			n += 3
+		}
+		if t0 != tl {
+			buf[n] = Separator
+			copy(buf[n+1:], targ[t0:])
+		}
+		return string(buf), nil
+	}
+	return targ[t0:], nil
+}
+
 // SkipDir is used as a return value from WalkFuncs to indicate that
 // the directory named in the call is to be skipped. It is not returned
 // as an error by any function.
diff --git a/src/pkg/path/filepath/path_test.go b/src/pkg/path/filepath/path_test.go
index 850ead8e812..9234c0df6c6 100644
--- a/src/pkg/path/filepath/path_test.go
+++ b/src/pkg/path/filepath/path_test.go
@@ -572,10 +572,11 @@ var abstests = []string{
 	"pkg/../../AUTHORS",
 	"Make.pkg",
 	"pkg/Makefile",
-
-	// Already absolute
+	".",
 	"$GOROOT/src/Make.pkg",
 	"$GOROOT/src/../src/Make.pkg",
+	"$GOROOT/misc/cgo",
+	"$GOROOT",
 }
 
 func TestAbs(t *testing.T) {
@@ -589,14 +590,15 @@ func TestAbs(t *testing.T) {
 	os.Chdir(cwd)
 	for _, path := range abstests {
 		path = strings.Replace(path, "$GOROOT", goroot, -1)
-		abspath, err := filepath.Abs(path)
-		if err != nil {
-			t.Errorf("Abs(%q) error: %v", path, err)
-		}
 		info, err := os.Stat(path)
 		if err != nil {
 			t.Errorf("%s: %s", path, err)
 		}
+
+		abspath, err := filepath.Abs(path)
+		if err != nil {
+			t.Errorf("Abs(%q) error: %v", path, err)
+		}
 		absinfo, err := os.Stat(abspath)
 		if err != nil || absinfo.Ino != info.Ino {
 			t.Errorf("Abs(%q)=%q, not the same file", path, abspath)
@@ -610,6 +612,77 @@ func TestAbs(t *testing.T) {
 	}
 }
 
+type RelTests struct {
+	root, path, want string
+}
+
+var reltests = []RelTests{
+	{"a/b", "a/b", "."},
+	{"a/b/.", "a/b", "."},
+	{"a/b", "a/b/.", "."},
+	{"./a/b", "a/b", "."},
+	{"a/b", "./a/b", "."},
+	{"ab/cd", "ab/cde", "../cde"},
+	{"ab/cd", "ab/c", "../c"},
+	{"a/b", "a/b/c/d", "c/d"},
+	{"a/b", "a/b/../c", "../c"},
+	{"a/b/../c", "a/b", "../b"},
+	{"a/b/c", "a/c/d", "../../c/d"},
+	{"a/b", "c/d", "../../c/d"},
+	{"../../a/b", "../../a/b/c/d", "c/d"},
+	{"/a/b", "/a/b", "."},
+	{"/a/b/.", "/a/b", "."},
+	{"/a/b", "/a/b/.", "."},
+	{"/ab/cd", "/ab/cde", "../cde"},
+	{"/ab/cd", "/ab/c", "../c"},
+	{"/a/b", "/a/b/c/d", "c/d"},
+	{"/a/b", "/a/b/../c", "../c"},
+	{"/a/b/../c", "/a/b", "../b"},
+	{"/a/b/c", "/a/c/d", "../../c/d"},
+	{"/a/b", "/c/d", "../../c/d"},
+	{"/../../a/b", "/../../a/b/c/d", "c/d"},
+	{".", "a/b", "a/b"},
+	{".", "..", ".."},
+
+	// can't do purely lexically
+	{"..", ".", "err"},
+	{"..", "a", "err"},
+	{"../..", "..", "err"},
+	{"a", "/a", "err"},
+	{"/a", "a", "err"},
+}
+
+var winreltests = []RelTests{
+	{`C:a\b\c`, `C:a/b/d`, `..\d`},
+	{`C:\`, `D:\`, `err`},
+	{`C:`, `D:`, `err`},
+}
+
+func TestRel(t *testing.T) {
+	tests := append([]RelTests{}, reltests...)
+	if runtime.GOOS == "windows" {
+		for i := range tests {
+			tests[i].want = filepath.FromSlash(tests[i].want)
+		}
+		tests = append(tests, winreltests...)
+	}
+	for _, test := range tests {
+		got, err := filepath.Rel(test.root, test.path)
+		if test.want == "err" {
+			if err == nil {
+				t.Errorf("Rel(%q, %q)=%q, want error", test.root, test.path, got)
+			}
+			continue
+		}
+		if err != nil {
+			t.Errorf("Rel(%q, %q): want %q, got error: %s", test.root, test.path, test.want, err)
+		}
+		if got != test.want {
+			t.Errorf("Rel(%q, %q)=%q, want %q", test.root, test.path, got, test.want)
+		}
+	}
+}
+
 type VolumeNameTest struct {
 	path string
 	vol  string