diff --git a/polygerrit-ui/app/elements/change/gr-label-score-row/gr-label-score-row.js b/polygerrit-ui/app/elements/change/gr-label-score-row/gr-label-score-row.js
index d5506ad..20fd147 100644
--- a/polygerrit-ui/app/elements/change/gr-label-score-row/gr-label-score-row.js
+++ b/polygerrit-ui/app/elements/change/gr-label-score-row/gr-label-score-row.js
@@ -16,6 +16,13 @@
 
   Polymer({
     is: 'gr-label-score-row',
+
+    /**
+     * Fired when any label is changed.
+     *
+     * @event labels-changed
+     */
+
     properties: {
       /**
        * @type {{ name: string }}
@@ -96,6 +103,7 @@
       // nothing and then to the new item.
       if (!e.target.selectedItem) { return; }
       this._selectedValueText = e.target.selectedItem.getAttribute('title');
+      this.fire('labels-changed');
     },
 
     _computeAnyPermittedLabelValues(permittedLabels, label) {
diff --git a/polygerrit-ui/app/elements/change/gr-label-score-row/gr-label-score-row_test.html b/polygerrit-ui/app/elements/change/gr-label-score-row/gr-label-score-row_test.html
index 537bd25..da450e1 100644
--- a/polygerrit-ui/app/elements/change/gr-label-score-row/gr-label-score-row_test.html
+++ b/polygerrit-ui/app/elements/change/gr-label-score-row/gr-label-score-row_test.html
@@ -103,6 +103,8 @@
     });
 
     test('label picker', () => {
+      const labelsChangedHandler = sandbox.stub();
+      element.addEventListener('labels-changed', labelsChangedHandler);
       assert.ok(element.$$('iron-selector'));
       MockInteractions.tap(element.$$(
           'gr-button[value="-1"]'));
@@ -112,6 +114,7 @@
           .textContent.trim(), '-1');
       assert.strictEqual(
           element.$.selectedValueLabel.textContent.trim(), 'bad');
+      assert.isTrue(labelsChangedHandler.called);
     });
 
     test('correct item is selected', () => {
diff --git a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.html b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.html
index 2f9f78c..24673ff 100644
--- a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.html
+++ b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.html
@@ -241,6 +241,7 @@
             id="labelScores"
             account="[[_account]]"
             change="[[change]]"
+            on-labels-changed="_handleLabelsChanged"
             permitted-labels=[[permittedLabels]]></gr-label-scores>
       </section>
       <section class="draftsContainer" hidden$="[[_computeHideDraftList(diffDrafts)]]">
@@ -265,7 +266,7 @@
       <section>
         <gr-button
             primary
-            disabled="[[_isState(knownLatestState, 'not-latest')]]"
+            disabled="[[_computeSendButtonDisabled(knownLatestState, _sendButtonLabel, diffDrafts, draft, _reviewersMutated, _labelsChanged)]]"
             class="action send"
             on-tap="_sendTapHandler">[[_sendButtonLabel]]</gr-button>
         <template is="dom-if" if="[[canBeStarted]]">
diff --git a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.js b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.js
index 752f15a..9f4a79a 100644
--- a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.js
+++ b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.js
@@ -34,6 +34,11 @@
     NOT_LATEST: 'not-latest',
   };
 
+  const ButtonLabels = {
+    START_REVIEW: 'Start review',
+    SEND: 'Send',
+  };
+
   // TODO(logan): Remove once the fix for issue 6841 is stable on
   // googlesource.com.
   const START_REVIEW_MESSAGE = 'This change is ready for review.';
@@ -175,6 +180,14 @@
         computed: '_computeCCsEnabled(serverConfig)',
       },
       _savingComments: Boolean,
+      _reviewersMutated: {
+        type: Boolean,
+        value: false,
+      },
+      _labelsChanged: {
+        type: Boolean,
+        value: false,
+      },
     },
 
     FocusTarget,
@@ -268,12 +281,14 @@
 
     _ccsChanged(splices) {
       if (splices && splices.indexSplices) {
+        this._reviewersMutated = true;
         this._processReviewerChange(splices.indexSplices, ReviewerTypes.CC);
       }
     },
 
     _reviewersChanged(splices) {
       if (splices && splices.indexSplices) {
+        this._reviewersMutated = true;
         this._processReviewerChange(splices.indexSplices,
             ReviewerTypes.REVIEWER);
         let key;
@@ -768,6 +783,11 @@
       });
     },
 
+    _handleLabelsChanged() {
+      this._labelsChanged = Object.keys(
+          this.$.labelScores.getLabelValues()).length !== 0;
+    },
+
     _isState(knownLatestState, value) {
       return knownLatestState === value;
     },
@@ -778,7 +798,7 @@
     },
 
     _computeSendButtonLabel(canBeStarted) {
-      return canBeStarted ? 'Start review' : 'Send';
+      return canBeStarted ? ButtonLabels.START_REVIEW : ButtonLabels.SEND;
     },
 
     _computeCCsEnabled(serverConfig) {
@@ -788,5 +808,17 @@
     _computeSavingLabelClass(savingComments) {
       return savingComments ? 'saving' : '';
     },
+
+    _computeSendButtonDisabled(knownLatestState, buttonLabel, drafts, text,
+        reviewersMutated, labelsChanged) {
+      if (this._isState(knownLatestState, LatestPatchState.NOT_LATEST)) {
+        return true;
+      }
+      if (buttonLabel === ButtonLabels.START_REVIEW) {
+        return false;
+      }
+      return !(drafts.length || text.length || reviewersMutated ||
+          labelsChanged);
+    },
   });
 })();
diff --git a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog_test.html b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog_test.html
index 0f90019..9a83259 100644
--- a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog_test.html
+++ b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog_test.html
@@ -55,6 +55,8 @@
       stub('gr-rest-api-interface', {
         getConfig() { return Promise.resolve({}); },
         getAccount() { return Promise.resolve({}); },
+        getChange() { return Promise.resolve({}); },
+        getChangeSuggestedReviewers() { return Promise.resolve([]); },
       });
 
       element = fixture('basic');
@@ -1007,5 +1009,20 @@
         });
       });
     });
+
+    test('_computeSendButtonDisabled', () => {
+      const fn = element._computeSendButtonDisabled.bind(element);
+      assert.isTrue(fn('not-latest'));
+      assert.isFalse(fn('latest', 'Start review'));
+      assert.isTrue(fn('latest', 'Send', [], '', false, false));
+      // Mock nonempty comment draft array.
+      assert.isFalse(fn('latest', 'Send', ['test'], '', false, false));
+      // Mock nonempty change message.
+      assert.isFalse(fn('latest', 'Send', [], 'test', false, false));
+      // Mock reviewers mutated.
+      assert.isFalse(fn('latest', 'Send', [], '', true, false));
+      // Mock labels changed.
+      assert.isFalse(fn('latest', 'Send', [], '', false, true));
+    });
   });
 </script>