diff --git a/docs/changes.txt b/docs/changes.txt
index 3319dd013b..ebbe812abc 100644
--- a/docs/changes.txt
+++ b/docs/changes.txt
@@ -480,6 +480,7 @@ All:
 All (GUI):
 
 - Added strike-through support to wxFont (Igor Korot).
+- Add support for horizontal mouse wheel events to MSW and GTK (Lauri Nurmi).
 - Fix infinite loop in wxHtmlEasyPrinting when trying to page break images
   taller than the page height in wxHtmlEasyPrinting (Laurent Poujoulat).
 - Added wxFilePickerCtrl::SetInitialDirectory().
@@ -521,7 +522,6 @@ MSW:
 - Add wxActiveXContainer::QueryClientSiteInterface and implement it in
   wxWebViewIE to improve the default behaviour (Allonii).
 - Update stretchable spaces in wxToolBar after tool removal (Catalin Raceanu).
-- Add support for horizontal mouse wheel events (Lauri Nurmi).
 - Implement wxGraphicsContext::SetInterpolationQuality() (Eric Jensen).
 - Fix coordinate handling in wxDC::Blit() when source DC is a DIB.
 - Fix handling of composite windows in wxToolTip (Armel Asselin).
diff --git a/src/gtk/window.cpp b/src/gtk/window.cpp
index b8734c412d..ac10b6fc7d 100644
--- a/src/gtk/window.cpp
+++ b/src/gtk/window.cpp
@@ -1641,22 +1641,43 @@ window_scroll_event_hscrollbar(GtkWidget*, GdkEventScroll* gdk_event, wxWindow*
 static gboolean
 window_scroll_event(GtkWidget*, GdkEventScroll* gdk_event, wxWindow* win)
 {
-    if (gdk_event->direction != GDK_SCROLL_UP &&
-        gdk_event->direction != GDK_SCROLL_DOWN)
-    {
-        return false;
-    }
-
     wxMouseEvent event(wxEVT_MOUSEWHEEL);
     InitMouseEvent(win, event, gdk_event);
 
     // FIXME: Get these values from GTK or GDK
     event.m_linesPerAction = 3;
     event.m_wheelDelta = 120;
-    if (gdk_event->direction == GDK_SCROLL_UP)
-        event.m_wheelRotation = 120;
-    else
-        event.m_wheelRotation = -120;
+
+    // Determine the scroll direction.
+    switch (gdk_event->direction)
+    {
+        case GDK_SCROLL_UP:
+        case GDK_SCROLL_RIGHT:
+            event.m_wheelRotation = 120;
+            break;
+
+        case GDK_SCROLL_DOWN:
+        case GDK_SCROLL_LEFT:
+            event.m_wheelRotation = -120;
+            break;
+
+        default:
+            return false;  // Unknown/unhandled direction
+    }
+
+    // And the scroll axis.
+    switch (gdk_event->direction)
+    {
+        case GDK_SCROLL_UP:
+        case GDK_SCROLL_DOWN:
+            event.m_wheelAxis = wxMOUSE_WHEEL_VERTICAL;
+            break;
+
+        case GDK_SCROLL_LEFT:
+        case GDK_SCROLL_RIGHT:
+            event.m_wheelAxis = wxMOUSE_WHEEL_HORIZONTAL;
+            break;
+    }
 
     if (win->GTKProcessEvent(event))
       return TRUE;