diff --git a/doc/decision-tree-widget/tree.yml b/doc/decision-tree-widget/tree.yml
index 339d4aa606..1b49884565 100644
--- a/doc/decision-tree-widget/tree.yml
+++ b/doc/decision-tree-widget/tree.yml
@@ -77,9 +77,6 @@ children:
           - label: emitted just before the current value
             children:
             - label: distinctUntilKeyChanged
-          - label: emitted some time in the past
-            children:
-            - label: distinctKey
       - label: that occur too frequently
         children:
         - label: by emitting the first value in each time window
diff --git a/src/add/operator/distinctKey.ts b/src/add/operator/distinctKey.ts
deleted file mode 100644
index 76544ed2fd..0000000000
--- a/src/add/operator/distinctKey.ts
+++ /dev/null
@@ -1,10 +0,0 @@
-import { Observable } from '../../Observable';
-import { distinctKey } from '../../operator/distinctKey';
-
-Observable.prototype.distinctKey = distinctKey;
-
-declare module '../../Observable' {
-  interface Observable<T> {
-    distinctKey: typeof distinctKey;
-  }
-}
\ No newline at end of file
diff --git a/src/operator/filter.ts b/src/operator/filter.ts
index a227a9e19c..be3f371eaa 100644
--- a/src/operator/filter.ts
+++ b/src/operator/filter.ts
@@ -30,7 +30,6 @@ export function filter<T, S extends T>(this: Observable<T>,
  * clicksOnDivs.subscribe(x => console.log(x));
  *
  * @see {@link distinct}
- * @see {@link distinctKey}
  * @see {@link distinctUntilChanged}
  * @see {@link distinctUntilKeyChanged}
  * @see {@link ignoreElements}