diff --git a/src/cmd/go/pkg.go b/src/cmd/go/pkg.go
index c855fa6c4c..718b9fea03 100644
--- a/src/cmd/go/pkg.go
+++ b/src/cmd/go/pkg.go
@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"fmt"
 	"go/build"
+	"go/doc"
 	"go/scanner"
 	"os"
 	"path/filepath"
@@ -220,32 +221,6 @@ func reusePackage(p *Package, stk *importStack) *Package {
 	return p
 }
 
-// firstSentence returns the first sentence of the document text.
-// The sentence ends after the first period followed by a space.
-// The returned sentence will have no \n \r or \t characters and
-// will use only single spaces between words.
-func firstSentence(text string) string {
-	var b []byte
-	space := true
-Loop:
-	for i := 0; i < len(text); i++ {
-		switch c := text[i]; c {
-		case ' ', '\t', '\r', '\n':
-			if !space {
-				space = true
-				if len(b) > 0 && b[len(b)-1] == '.' {
-					break Loop
-				}
-				b = append(b, ' ')
-			}
-		default:
-			space = false
-			b = append(b, c)
-		}
-	}
-	return string(b)
-}
-
 // isGoTool is the list of directories for Go programs that are installed in
 // $GOROOT/bin/tool.
 var isGoTool = map[string]bool{
@@ -298,7 +273,7 @@ func scanPackage(ctxt *build.Context, t *build.Tree, arg, importPath, dir string
 
 	p.info = info
 	p.Name = info.Package
-	p.Doc = firstSentence(info.PackageComment.Text())
+	p.Doc = doc.Synopsis(info.PackageComment.Text())
 	p.Imports = info.Imports
 	p.GoFiles = info.GoFiles
 	p.TestGoFiles = info.TestGoFiles
diff --git a/src/cmd/godoc/dirtrees.go b/src/cmd/godoc/dirtrees.go
index c61f791dcb..90f2c80ce7 100644
--- a/src/cmd/godoc/dirtrees.go
+++ b/src/cmd/godoc/dirtrees.go
@@ -8,6 +8,7 @@ package main
 
 import (
 	"bytes"
+	"go/doc"
 	"go/parser"
 	"go/token"
 	"log"
@@ -135,7 +136,7 @@ func (b *treeBuilder) newDirTree(fset *token.FileSet, path, name string, depth i
 							i = 3 // none of the above
 						}
 						if 0 <= i && i < len(synopses) && synopses[i] == "" {
-							synopses[i] = firstSentence(file.Doc.Text())
+							synopses[i] = doc.Synopsis(file.Doc.Text())
 						}
 					}
 				}
diff --git a/src/pkg/go/doc/synopsis.go b/src/pkg/go/doc/synopsis.go
new file mode 100644
index 0000000000..2192d78c0c
--- /dev/null
+++ b/src/pkg/go/doc/synopsis.go
@@ -0,0 +1,52 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import "unicode"
+
+// firstSentenceLen returns the length of the first sentence in s.
+// The sentence ends after the first period followed by space and
+// not preceded by exactly one uppercase letter.
+//
+func firstSentenceLen(s string) int {
+	var ppp, pp, p rune
+	for i, q := range s {
+		if q == '\n' || q == '\r' || q == '\t' {
+			q = ' '
+		}
+		if q == ' ' && p == '.' && (!unicode.IsUpper(pp) || unicode.IsUpper(ppp)) {
+			return i
+		}
+		ppp, pp, p = pp, p, q
+	}
+	return len(s)
+}
+
+// Synopsis returns a cleaned version of the first sentence in s.
+// That sentence ends after the first period followed by space and
+// not preceded by exactly one uppercase letter. The result string
+// has no \n, \r, or \t characters and uses only single spaces between
+// words.
+//
+func Synopsis(s string) string {
+	n := firstSentenceLen(s)
+	var b []byte
+	p := byte(' ')
+	for i := 0; i < n; i++ {
+		q := s[i]
+		if q == '\n' || q == '\r' || q == '\t' {
+			q = ' '
+		}
+		if q != ' ' || p != ' ' {
+			b = append(b, q)
+			p = q
+		}
+	}
+	// remove trailing blank, if any
+	if n := len(b); n > 0 && p == ' ' {
+		b = b[0 : n-1]
+	}
+	return string(b)
+}
diff --git a/src/pkg/go/doc/synopsis_test.go b/src/pkg/go/doc/synopsis_test.go
new file mode 100644
index 0000000000..dfc6598af4
--- /dev/null
+++ b/src/pkg/go/doc/synopsis_test.go
@@ -0,0 +1,44 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package doc
+
+import "testing"
+
+var tests = []struct {
+	txt string
+	fsl int
+	syn string
+}{
+	{"", 0, ""},
+	{"foo", 3, "foo"},
+	{"foo.", 4, "foo."},
+	{"foo.bar", 7, "foo.bar"},
+	{"  foo.  ", 6, "foo."},
+	{"  foo\t  bar.\n", 12, "foo bar."},
+	{"  foo\t  bar.\n", 12, "foo bar."},
+	{"a  b\n\nc\r\rd\t\t", 12, "a b c d"},
+	{"a  b\n\nc\r\rd\t\t  . BLA", 15, "a b c d ."},
+	{"Package poems by T.S.Eliot. To rhyme...", 27, "Package poems by T.S.Eliot."},
+	{"Package poems by T. S. Eliot. To rhyme...", 29, "Package poems by T. S. Eliot."},
+	{"foo implements the foo ABI. The foo ABI is...", 27, "foo implements the foo ABI."},
+	{"Package\nfoo. ..", 12, "Package foo."},
+	{"P . Q.", 3, "P ."},
+	{"P. Q.   ", 8, "P. Q."},
+	{"Package Καλημέρα κόσμε.", 36, "Package Καλημέρα κόσμε."},
+	{"Package こんにちは 世界\n", 31, "Package こんにちは 世界"},
+}
+
+func TestSynopsis(t *testing.T) {
+	for _, e := range tests {
+		fsl := firstSentenceLen(e.txt)
+		if fsl != e.fsl {
+			t.Errorf("got fsl = %d; want %d for %q\n", fsl, e.fsl, e.txt)
+		}
+		syn := Synopsis(e.txt)
+		if syn != e.syn {
+			t.Errorf("got syn = %q; want %q for %q\n", syn, e.syn, e.txt)
+		}
+	}
+}