diff --git a/mozilla/calendar/lightning/content/calendar-todo-list.xml b/mozilla/calendar/lightning/content/calendar-todo-list.xml
index 5ec57f1f1d1..e6f22e87c7a 100644
--- a/mozilla/calendar/lightning/content/calendar-todo-list.xml
+++ b/mozilla/calendar/lightning/content/calendar-todo-list.xml
@@ -52,6 +52,7 @@
         ]]></destructor>
 
         <field name="mCalendar">null</field>
+        <field name="mShowCompleted">true</field>
         <field name="mOperationListener"><![CDATA[
         ({
             todoList: this,
@@ -167,6 +168,14 @@
             ]]></setter>
         </property>
 
+        <property name="showCompleted">
+            <getter><![CDATA[ return this.mShowCompleted; ]]></getter>
+            <setter><![CDATA[
+                this.mShowCompleted = val;
+                this.rebuildDisplay();
+            ]]></setter>
+        </property>
+
         <method name="fireEvent">
             <parameter name="aEventType"/>
             <body><![CDATA[
@@ -207,7 +216,12 @@
                 if (!this.mCalendar)
                     return;
                 const cIC = Components.interfaces.calICalendar;
-                var filter = cIC.ITEM_FILTER_TYPE_TODO | cIC.ITEM_FILTER_COMPLETED_ALL;
+                var filter = cIC.ITEM_FILTER_TYPE_TODO;
+                if (this.mShowCompleted) {
+                    filter |= cIC.ITEM_FILTER_COMPLETED_ALL;
+                } else {
+                    filter |= cIC.ITEM_FILTER_COMPLETED_NO;
+                }
                 this.mCalendar.getItems(filter, 0, null, null, this.mOperationListener);
             ]]></body>
         </method>
diff --git a/mozilla/calendar/lightning/content/todo-list-overlay.xul b/mozilla/calendar/lightning/content/todo-list-overlay.xul
index 9792efd7fb2..c74a6dbf35c 100644
--- a/mozilla/calendar/lightning/content/todo-list-overlay.xul
+++ b/mozilla/calendar/lightning/content/todo-list-overlay.xul
@@ -45,6 +45,10 @@
 
 <script type="application/x-javascript" src="chrome://lightning/content/todo-list.js"/>
 <vbox id="todo-tab-panel" flex="1">
+  <checkbox id="completed-tasks-checkbox"
+            label="&lightning.sidebar.showCompletedTasks.label;"
+            persist="checked"
+            oncommand="toggleCompletedTasks()" />
   <calendar-todo-list id="calendar-todo-list" flex="1"/>
 </vbox>
 </overlay>
diff --git a/mozilla/calendar/lightning/content/todo-list.js b/mozilla/calendar/lightning/content/todo-list.js
index 0b395ab6863..76196cb6137 100644
--- a/mozilla/calendar/lightning/content/todo-list.js
+++ b/mozilla/calendar/lightning/content/todo-list.js
@@ -70,6 +70,15 @@ function initializeTodoList()
     todoList.addEventListener("todo-item-open", editTodoItem, false);
     todoList.addEventListener("todo-item-delete", deleteTodoItem, false);
     todoList.addEventListener("todo-empty-dblclick", newTodoItem, false);
+    todoList.showCompleted = document.getElementById("completed-tasks-checkbox").checked;
+    return;
+}
+
+function toggleCompletedTasks()
+{
+    document.getElementById("calendar-todo-list").showCompleted =
+        !document.getElementById("calendar-todo-list").showCompleted;
+    return;
 }
 
 window.addEventListener("load", initializeTodoList, false);
diff --git a/mozilla/calendar/locales/en-US/chrome/lightning/lightning.dtd b/mozilla/calendar/locales/en-US/chrome/lightning/lightning.dtd
index 1e463f9db23..28b67088c67 100644
--- a/mozilla/calendar/locales/en-US/chrome/lightning/lightning.dtd
+++ b/mozilla/calendar/locales/en-US/chrome/lightning/lightning.dtd
@@ -69,6 +69,7 @@
 <!ENTITY lightning.sidebar.edit.label        "Edit…">
 <!ENTITY lightning.sidebar.delete.label      "Delete">
 <!ENTITY lightning.calendar.label            "Calendar">
+<!ENTITY lightning.sidebar.showCompletedTasks.label "Show completed tasks">
 
 <!-- Messenger Toolbar -->
 <!ENTITY lightning.toolbar.dayview.label     "Day View">
