diff --git a/datatype/labelmap/handlers.go b/datatype/labelmap/handlers.go
index abb1dd86..3e5af0d9 100644
--- a/datatype/labelmap/handlers.go
+++ b/datatype/labelmap/handlers.go
@@ -433,6 +433,19 @@ func (d *Data) handleIndex(ctx *datastore.VersionedCtx, w http.ResponseWriter, r
 	timedLog.Infof("HTTP %s index for label %d (%s)", r.Method, label, r.URL)
 }
 
+func (d *Data) handleExistingLabels(ctx *datastore.VersionedCtx, w http.ResponseWriter, r *http.Request) {
+	// GET <api URL>/node/<UUID>/<data name>/existing-labels
+
+	if strings.ToLower(r.Method) != "get" {
+		server.BadRequest(w, r, "only GET action allowed for /existing-labels endpoint")
+		return
+	}
+
+	if err := d.writeExistingIndices(ctx, w, nil); err != nil {
+		dvid.Errorf("unable to write existing labels: %v", err)
+	}
+}
+
 func (d *Data) handleListLabels(ctx *datastore.VersionedCtx, w http.ResponseWriter, r *http.Request) {
 	// GET <api URL>/node/<UUID>/<data name>/listlabels
 	const MaxReturnedLabels = 10000000
diff --git a/datatype/labelmap/labelidx.go b/datatype/labelmap/labelidx.go
index ae00a36b..f924dea8 100644
--- a/datatype/labelmap/labelidx.go
+++ b/datatype/labelmap/labelidx.go
@@ -1765,6 +1765,62 @@ func (d *Data) indexThread(f *os.File, mu *sync.Mutex, wg *sync.WaitGroup, chunk
 	}
 }
 
+// scan all label indices for the presence of its key, writing existing key's body id
+// into a stream and/or setting it in an internal map.
+func (d *Data) writeExistingIndices(ctx *datastore.VersionedCtx, w http.ResponseWriter, bodymap map[uint64]struct{}) error {
+	timedLog := dvid.NewTimeLog()
+
+	store, err := datastore.GetOrderedKeyValueDB(d)
+	if err != nil {
+		return fmt.Errorf("problem getting store for data %q: %v", d.DataName(), err)
+	}
+
+	minTKey := NewLabelIndexTKey(0)
+	maxTKey := NewLabelIndexTKey(math.MaxUint64)
+	keyChan := make(storage.KeyChan)
+	go func() {
+		store.SendKeysInRange(ctx, minTKey, maxTKey, keyChan)
+		close(keyChan)
+	}()
+
+	if w != nil {
+		w.Header().Set("Content-type", "application/json")
+		fmt.Fprintf(w, "[")
+	}
+
+	var numExist uint64
+	for key := range keyChan {
+		if key != nil {
+			tkey, err := storage.TKeyFromKey(key)
+			if err != nil {
+				dvid.Errorf("Unable to get TKey from key %v for data %q: %v\n", key, d.DataName(), err)
+				continue
+			}
+			label, err := DecodeLabelIndexTKey(tkey)
+			if err != nil {
+				dvid.Errorf("Couldn't decode label index key %v for data %q\n", key, d.DataName())
+				continue
+			}
+			if bodymap != nil {
+				bodymap[label] = struct{}{}
+			}
+			if w != nil {
+				if numExist > 0 {
+					fmt.Fprintf(w, ",")
+				}
+				fmt.Fprintf(w, "%d", label)
+			}
+			numExist++
+		}
+	}
+	if w != nil {
+		fmt.Fprintf(w, "]")
+	}
+
+	timedLog.Infof("Finished writing %d existing label indices.", numExist)
+	return nil
+}
+
 // scan all label indices in this labelmap instance, writing Blocks data into a given file
 func (d *Data) writeIndices(f *os.File, outPath string, v dvid.VersionID) {
 	timedLog := dvid.NewTimeLog()
diff --git a/datatype/labelmap/labelmap.go b/datatype/labelmap/labelmap.go
index 82dafb87..e7eefabe 100644
--- a/datatype/labelmap/labelmap.go
+++ b/datatype/labelmap/labelmap.go
@@ -1484,6 +1484,15 @@ POST <api URL>/node/<UUID>/<data name>/index/<label>
 		   a 404 not found is returned.
 
 	
+GET <api URL>/node/<UUID>/<data name>/existing-labels
+
+	Streams JSON list of all existing labels in the most computationally efficient manner,
+	particularly when backed by a Badger key-value store or other backend that has fast
+	key scans.
+
+	Note that because the data is streamed over HTTP, an error code cannot be sent once data is 
+	in flight.
+
 GET <api URL>/node/<UUID>/<data name>/listlabels[?queryopts]
 
 	Streams labels and optionally their voxel counts in numerical order up to a maximum
@@ -2886,6 +2895,9 @@ func (d *Data) ServeHTTP(uuid dvid.UUID, ctx *datastore.VersionedCtx, w http.Res
 	case "labels":
 		d.handleLabels(ctx, w, r)
 
+	case "existing-labels":
+		d.handleExistingLabels(ctx, w, r)
+
 	case "listlabels":
 		d.handleListLabels(ctx, w, r)
 
diff --git a/datatype/labelmap/labelmap_test.go b/datatype/labelmap/labelmap_test.go
index 7b24078c..c7b2e142 100644
--- a/datatype/labelmap/labelmap_test.go
+++ b/datatype/labelmap/labelmap_test.go
@@ -1261,12 +1261,19 @@ func TestMultiscaleIngest(t *testing.T) {
 	server.TestBadHTTP(t, "GET", reqStr, nil)
 
 	reqStr = fmt.Sprintf("%snode/%s/labels/sizes", server.WebAPIPath, uuid)
-	bodystr := "[1, 2, 3, 13, 209, 311]"
+	bodystr := "[1,2,3,13,209,311]"
 	r := server.TestHTTP(t, "GET", reqStr, bytes.NewBufferString(bodystr))
 	if string(r) != "[64000,64000,0,64000,64000,64000]" {
 		t.Errorf("bad batch sizes result.  got: %s\n", string(r))
 	}
 
+	// Verify the existing label streaming endpoint.
+	reqStr = fmt.Sprintf("%snode/%s/labels/existing-labels", server.WebAPIPath, uuid)
+	r = server.TestHTTP(t, "GET", reqStr, nil)
+	if string(r) != "[1,2,13,209,311]" {
+		t.Errorf("expected [1,2,13,209,311] from /existing-labels but got %s\n", string(r))
+	}
+
 	// Verify the label streaming endpoint.
 	reqStr = fmt.Sprintf("%snode/%s/labels/listlabels", server.WebAPIPath, uuid)
 	r = server.TestHTTP(t, "GET", reqStr, nil)
diff --git a/datatype/labelmap/mutate_test.go b/datatype/labelmap/mutate_test.go
index 93ea8b91..41c03881 100644
--- a/datatype/labelmap/mutate_test.go
+++ b/datatype/labelmap/mutate_test.go
@@ -2248,6 +2248,13 @@ func TestMergeCleave(t *testing.T) {
 		t.Errorf("Expected supervoxel 4 original voxels to remain.  Instead some were removed.\n")
 	}
 
+	// Verify using existing label streaming endpoint.
+	reqStr = fmt.Sprintf("%snode/%s/labels/existing-labels", server.WebAPIPath, uuid)
+	r = server.TestHTTP(t, "GET", reqStr, nil)
+	if string(r) != "[1,2,4]" {
+		t.Errorf("expected [1,2,4] from /existing-labels but got %s\n", string(r))
+	}
+
 	// Cleave supervoxel 3 out of label 4.
 	reqStr = fmt.Sprintf("%snode/%s/labels/cleave/4?u=mrsmith&app=myapp", server.WebAPIPath, uuid)
 	r = server.TestHTTP(t, "POST", reqStr, bytes.NewBufferString("[3]"))