diff --git a/ChangeLog b/ChangeLog
index 1d60ea1c2..7ddc3d41f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,10 @@
 2011-01-20  Moritz Bunkus  <moritz@bunkus.org>
 
+	* mmg: enhancement: The output file name extension is
+	automatically set to ".mk3d" if the stereo mode parameter for any
+	video track is changed to anything else than "mono" or the default
+	value.
+
 	* mmg: enhancement: Added ".mk3d" to the list of known file name
 	extensions for Matroska files.
 
diff --git a/src/mmg/mmg_dialog.cpp b/src/mmg/mmg_dialog.cpp
index 323028af4..7429e7185 100644
--- a/src/mmg/mmg_dialog.cpp
+++ b/src/mmg/mmg_dialog.cpp
@@ -1326,17 +1326,20 @@ mmg_dialog::set_output_maybe(const wxString &new_output) {
 
 wxString
 mmg_dialog::suggest_file_name_extension() {
-  bool has_video = false, has_audio = false;
+  bool has_video = false, has_audio = false, has_stereo_mode = false;
 
   foreach(mmg_track_t *t, tracks) {
     if (t->is_video()) {
       has_video = true;
-      break;
+      if (t->stereo_mode >= 2)
+        has_stereo_mode = true;
+
     } else if (t->is_audio())
       has_audio = true;
   }
 
   return global_page->cb_webm_mode->IsChecked() ? wxU("webm")
+       : has_stereo_mode                        ? wxU("mk3d")
        : has_video                              ? wxU("mkv")
        : has_audio                              ? wxU("mka")
        :                                          wxU("mks");
diff --git a/src/mmg/tabs/input_format.cpp b/src/mmg/tabs/input_format.cpp
index f0055f3df..ecddd494e 100644
--- a/src/mmg/tabs/input_format.cpp
+++ b/src/mmg/tabs/input_format.cpp
@@ -400,7 +400,9 @@ tab_input_format::on_stereo_mode_changed(wxCommandEvent &evt) {
   if (input->dont_copy_values_now || (input->selected_track == -1))
     return;
 
-  tracks[input->selected_track]->stereo_mode = cob_stereo_mode->GetSelection();
+  mmg_track_t *track = tracks[input->selected_track];
+  track->stereo_mode = cob_stereo_mode->GetSelection();
+  mdlg->set_output_maybe(files[track->source]->file_name);
 }
 
 void