diff -up fltk-1.3.x-r8732/src/Fl_win32.cxx.hwheel fltk-1.3.x-r8732/src/Fl_win32.cxx
--- fltk-1.3.x-r8732/src/Fl_win32.cxx.hwheel	2011-05-21 23:55:59.000000000 +0200
+++ fltk-1.3.x-r8732/src/Fl_win32.cxx	2011-05-24 13:37:31.109341034 +0200
@@ -1125,12 +1286,28 @@ static LRESULT CALLBACK WndProc(HWND hWn
   case WM_MOUSEWHEEL: {
     static int delta = 0; // running total of all motion
     delta += (SHORT)(HIWORD(wParam));
+    Fl::e_dx = 0;
     Fl::e_dy = -delta / WHEEL_DELTA;
     delta += Fl::e_dy * WHEEL_DELTA;
     if (Fl::e_dy) Fl::handle(FL_MOUSEWHEEL, window);
     return 0;
   }
 
+// This is only defined on Vista and upwards...
+#ifndef WM_MOUSEHWHEEL
+#define WM_MOUSEHWHEEL 0x020E
+#endif
+
+  case WM_MOUSEHWHEEL: {
+    static int delta = 0; // running total of all motion
+    delta += (SHORT)(HIWORD(wParam));
+    Fl::e_dy = 0;
+    Fl::e_dx = delta / WHEEL_DELTA;
+    delta -= Fl::e_dx * WHEEL_DELTA;
+    if (Fl::e_dx) Fl::handle(FL_MOUSEWHEEL, window);
+    return 0;
+  }
+
   case WM_GETMINMAXINFO:
     Fl_X::i(window)->set_minmax((LPMINMAXINFO)lParam);
     break;
diff -up fltk-1.3.x-r8732/src/Fl_x.cxx.hwheel fltk-1.3.x-r8732/src/Fl_x.cxx
--- fltk-1.3.x-r8732/src/Fl_x.cxx.hwheel	2011-05-23 21:40:23.000000000 +0200
+++ fltk-1.3.x-r8732/src/Fl_x.cxx	2011-05-24 13:36:49.635113914 +0200
@@ -1441,12 +1608,19 @@ int fl_handle(const XEvent& thisevent)
   case ButtonPress:
     Fl::e_keysym = FL_Button + xevent.xbutton.button;
     set_event_xy();
+    Fl::e_dx = Fl::e_dy = 0;
     if (xevent.xbutton.button == Button4) {
       Fl::e_dy = -1; // Up
       event = FL_MOUSEWHEEL;
     } else if (xevent.xbutton.button == Button5) {
       Fl::e_dy = +1; // Down
       event = FL_MOUSEWHEEL;
+    } else if (xevent.xbutton.button == 6) {
+      Fl::e_dx = -1; // Left
+      event = FL_MOUSEWHEEL;
+    } else if (xevent.xbutton.button == 7) {
+      Fl::e_dx = +1; // Right
+      event = FL_MOUSEWHEEL;
     } else {
       Fl::e_state |= (FL_BUTTON1 << (xevent.xbutton.button-1));
       event = FL_PUSH;