diff --git a/src/lib/unsafe-html.ts b/src/lib/unsafe-html.ts
index 3ef7700ce8..d4c3e5668e 100644
--- a/src/lib/unsafe-html.ts
+++ b/src/lib/unsafe-html.ts
@@ -12,7 +12,7 @@
  * http://polymer.github.io/PATENTS.txt
  */
 
-import {directive, DirectiveFn, NodePart} from '../lit-html.js';
+import {directive, DirectiveFn, NodePart, _isPrimitiveValue} from '../lit-html.js';
 
 /**
  * Renders the result as HTML, rather than text.
@@ -23,7 +23,11 @@ import {directive, DirectiveFn, NodePart} from '../lit-html.js';
  */
 export const unsafeHTML = (value: any): DirectiveFn<NodePart> =>
     directive((part: NodePart): void => {
+      if (part._previousValue === value && _isPrimitiveValue(value)) {
+        return;
+      }
       const tmp = document.createElement('template');
       tmp.innerHTML = value;
       part.setValue(document.importNode(tmp.content, true));
+      part._previousValue = value;
     });
diff --git a/src/lit-html.ts b/src/lit-html.ts
index a74f76124f..7626da35ad 100644
--- a/src/lit-html.ts
+++ b/src/lit-html.ts
@@ -468,7 +468,7 @@ export const noChange = {};
  */
 export { noChange as directiveValue };
 
-const isPrimitiveValue = (value: any) => value === null ||
+export const _isPrimitiveValue = (value: any) => value === null ||
     !(typeof value === 'object' || typeof value === 'function');
 
 export interface Part {
@@ -531,7 +531,7 @@ export class AttributePart implements MultiPart {
     }
     for (let i = startIndex; i < startIndex + this.size; i++) {
       if (this._previousValues[i] !== values[i] ||
-          !isPrimitiveValue(values[i])) {
+          !_isPrimitiveValue(values[i])) {
         return false;
       }
     }
@@ -579,7 +579,7 @@ export class NodePart implements SinglePart {
     if (value === noChange) {
       return;
     }
-    if (isPrimitiveValue(value)) {
+    if (_isPrimitiveValue(value)) {
       // Handle primitive values
       // If the value didn't change, do nothing
       if (value === this._previousValue) {
diff --git a/src/test/lib/unsafe-html_test.ts b/src/test/lib/unsafe-html_test.ts
index dd65ab163f..05c668dcf8 100644
--- a/src/test/lib/unsafe-html_test.ts
+++ b/src/test/lib/unsafe-html_test.ts
@@ -24,8 +24,13 @@ const assert = chai.assert;
 
 suite('unsafeHTML', () => {
 
+  let container: HTMLElement;
+
+  setup(() => {
+    container = document.createElement('div');
+  });
+
   test('renders HTML', () => {
-    const container = document.createElement('div');
     render(
         html`<div>before${unsafeHTML('<span>inner</span>after</div>')}`,
         container);
@@ -33,4 +38,41 @@ suite('unsafeHTML', () => {
         stripExpressionDelimeters(container.innerHTML), '<div>before<span>inner</span>after</div>');
   });
 
+  test('dirty checks primitive values', () => {
+    const value = 'aaa';
+    const t = () => html`<div>${unsafeHTML(value)}</div>`;
+
+    // Initial render
+    render(t(), container);
+    assert.equal(stripExpressionDelimeters(container.innerHTML), '<div>aaa</div>');
+
+    // Modify instance directly. Since lit-html doesn't dirty check against
+    // actual DOM, but again previous part values, this modification should
+    // persist through the next render if dirty checking works.
+    const text = container.firstChild!.childNodes[1] as Text;
+    text.textContent = 'bbb';
+    assert.equal(stripExpressionDelimeters(container.innerHTML), '<div>bbb</div>');
+
+    // Re-render with the same value
+    render(t(), container);
+
+    assert.equal(stripExpressionDelimeters(container.innerHTML), '<div>bbb</div>');
+    const text2 = container.firstChild!.childNodes[1] as Text;
+    assert.strictEqual(text, text2);
+  });
+
+  test('does not dirty check complex values', () => {
+    const value = ['aaa'];
+    const t = () => html`<div>${unsafeHTML(value)}</div>`;
+
+    // Initial render
+    render(t(), container);
+    assert.equal(stripExpressionDelimeters(container.innerHTML), '<div>aaa</div>');
+
+    // Re-render with the same value, but a different deep property
+    value[0] = 'bbb';
+    render(t(), container);
+    assert.equal(stripExpressionDelimeters(container.innerHTML), '<div>bbb</div>');
+  });
+
 });