diff --git a/google-api-go-generator/gen.go b/google-api-go-generator/gen.go
index e4d1401..8278efa 100644
--- a/google-api-go-generator/gen.go
+++ b/google-api-go-generator/gen.go
@@ -61,6 +61,11 @@
 	serviceTypes = []string{"Service", "APIService"}
 )
 
+var (
+	errOldRevision = errors.New("revision pulled older than local cached revision")
+	errNoDoc       = errors.New("could not read discovery doc")
+)
+
 // API represents an API to generate, as well as its state while it's
 // generating.
 type API struct {
@@ -142,7 +147,10 @@
 		matches = append(matches, api)
 		log.Printf("Generating API %s", api.ID)
 		err := api.WriteGeneratedCode()
-		if err != nil && err != errNoDoc {
+		if err == errOldRevision {
+			log.Printf("Old revision found for %s, skipping generation", api.ID)
+			continue
+		} else if err != nil && err != errNoDoc {
 			errors = append(errors, &generateError{api, err})
 			continue
 		}
@@ -285,6 +293,40 @@
 	return a, nil
 }
 
+func checkAndUpdateSpecFile(file string, contents []byte) error {
+	// check if file exists
+	if _, err := os.Stat(file); os.IsNotExist(err) {
+		return writeFile(file, contents)
+	}
+	existing, err := ioutil.ReadFile(file)
+	if err != nil {
+		return err
+	}
+	if err := isNewerRevision(existing, contents); err != nil {
+		return err
+	}
+	return writeFile(file, contents)
+}
+
+// isNewerRevision returns nil if the contents of new has a newer revision than
+// the contents of old.
+func isNewerRevision(old []byte, new []byte) error {
+	type docRevision struct {
+		Revision string `json:"revision"`
+	}
+	var oldDoc, newDoc docRevision
+	if err := json.Unmarshal(old, &oldDoc); err != nil {
+		return err
+	}
+	if err := json.Unmarshal(new, &newDoc); err != nil {
+		return err
+	}
+	if newDoc.Revision < oldDoc.Revision {
+		return errOldRevision
+	}
+	return nil
+}
+
 func writeFile(file string, contents []byte) error {
 	// Don't write it if the contents are identical.
 	existing, err := ioutil.ReadFile(file)
@@ -496,10 +538,9 @@
 	return filepath.Join(a.SourceDir(), a.Package()+"-api.json")
 }
 
-var errNoDoc = errors.New("could not read discovery doc")
-
 // WriteGeneratedCode generates code for a.
-// It returns errNoDoc if we couldn't read the discovery doc.
+// It returns errNoDoc if we couldn't read the discovery doc or errOldRevision
+// if the API spec file being pulled in is older than the local cache.
 func (a *API) WriteGeneratedCode() error {
 	genfilename := *output
 	jsonBytes := a.jsonBytes()
@@ -509,7 +550,7 @@
 		return errNoDoc
 	}
 	if genfilename == "" {
-		if err := writeFile(a.JSONFile(), jsonBytes); err != nil {
+		if err := checkAndUpdateSpecFile(a.JSONFile(), jsonBytes); err != nil {
 			return err
 		}
 		outdir := a.SourceDir()
diff --git a/google-api-go-generator/gen_test.go b/google-api-go-generator/gen_test.go
index 092483d..1eda4b9 100644
--- a/google-api-go-generator/gen_test.go
+++ b/google-api-go-generator/gen_test.go
@@ -213,3 +213,32 @@
 		}
 	}
 }
+
+func TestIsNewerRevision(t *testing.T) {
+	olderBytesPath, newerBytesPath := filepath.Join("testdata", "rev20200415.json"), filepath.Join("testdata", "rev20200416.json")
+	olderBytes, err := ioutil.ReadFile(olderBytesPath)
+	if err != nil {
+		t.Fatalf("ioutil.ReadFile(%q) = %v; want nil", olderBytesPath, err)
+	}
+	newerBytes, err := ioutil.ReadFile(newerBytesPath)
+	if err != nil {
+		t.Fatalf("ioutil.ReadFile(%q) = %v; want nil", newerBytesPath, err)
+	}
+
+	// newBytes > oldBytes
+	if err := isNewerRevision(olderBytes, newerBytes); err != nil {
+		t.Fatalf("isNewerRevision(%q, %q) = %v; want nil", string(olderBytes), string(newerBytes), err)
+	}
+	// newBytes == newBytes
+	if err := isNewerRevision(newerBytes, newerBytes); err != nil {
+		t.Fatalf("isNewerRevision(%q, %q) = %v; want nil", string(newerBytes), string(newerBytes), err)
+	}
+	// newBytes < newBytes
+	err = isNewerRevision(newerBytes, olderBytes)
+	if err == nil {
+		t.Fatalf("isNewerRevision(%q, %q) = nil; want %v", string(newerBytes), string(olderBytes), errOldRevision)
+	}
+	if err != errOldRevision {
+		t.Fatalf("got %v, want %v", err, errOldRevision)
+	}
+}
diff --git a/google-api-go-generator/testdata/rev20200415.json b/google-api-go-generator/testdata/rev20200415.json
new file mode 100644
index 0000000..6b1c206
--- /dev/null
+++ b/google-api-go-generator/testdata/rev20200415.json
@@ -0,0 +1,3 @@
+{
+    "revision": "20200415"
+}
\ No newline at end of file
diff --git a/google-api-go-generator/testdata/rev20200416.json b/google-api-go-generator/testdata/rev20200416.json
new file mode 100644
index 0000000..c04f361
--- /dev/null
+++ b/google-api-go-generator/testdata/rev20200416.json
@@ -0,0 +1,3 @@
+{
+    "revision": "20200416"
+}
\ No newline at end of file