diff --git a/docs/rh-log.txt b/docs/rh-log.txt
index 2290e9e5e..eb67146e8 100644
--- a/docs/rh-log.txt
+++ b/docs/rh-log.txt
@@ -1,4 +1,8 @@
 June 5, 2009
+- Added support for horizontal mouse wheels, and set invprev/invnext as
+  default bindings for it. (Only works with Raw Input on XP. Vista has
+  support through Win32 mouse, as well. DirectInput does not support it at
+  all, because the DirectInput mouse device does not expose this control.)
 - Added a label for the F16 key and mapped the kp= key on a Mac keyboard to
   the PC98 equivalent so it will be identified as kp=. (Interestingly, F13-
   F16 and kp= only generate events when using Raw Input, not when using
diff --git a/src/c_bind.cpp b/src/c_bind.cpp
index 01456afb1..438f18eda 100644
--- a/src/c_bind.cpp
+++ b/src/c_bind.cpp
@@ -70,6 +70,8 @@ static const FBinding DefBindings[] =
 	{ "0", "slot 0" },
 	{ "[", "invprev" },
 	{ "]", "invnext" },
+	{ "mwheelleft", "invprev" },
+	{ "mwheelright", "invnext" },
 	{ "enter", "invuse" },
 	{ "-", "sizedown" },
 	{ "=", "sizeup" },
@@ -243,6 +245,7 @@ const char *KeyNames[NUM_KEYS] =
 	"pov4up",	"pov4right","pov4down",	"pov4left",		// Fourth POV hat
 
 	"mwheelup",	"mwheeldown",							// the mouse wheel
+	"mwheelright", "mwheelleft",
 };
 
 static FString Bindings[NUM_KEYS];
diff --git a/src/c_console.cpp b/src/c_console.cpp
index f1552d7ba..0cf2d9bce 100644
--- a/src/c_console.cpp
+++ b/src/c_console.cpp
@@ -1428,11 +1428,11 @@ static bool C_HandleKey (event_t *ev, BYTE *buffer, int len)
 	case EV_GUI_WheelDown:
 		if (!(ev->data3 & GKM_SHIFT))
 		{
-			data1 = GK_PGDN + ev->subtype - EV_GUI_WheelDown;
+			data1 = GK_PGDN + EV_GUI_WheelDown - ev->subtype;
 		}
 		else
 		{
-			data1 = GK_DOWN + ev->subtype - EV_GUI_WheelDown;
+			data1 = GK_DOWN + EV_GUI_WheelDown - ev->subtype;
 		}
 		// Intentional fallthrough
 
diff --git a/src/d_gui.h b/src/d_gui.h
index 30bd21cb6..5e9854f7c 100644
--- a/src/d_gui.h
+++ b/src/d_gui.h
@@ -53,8 +53,10 @@ enum EGUIEvent
 	EV_GUI_RButtonDown,
 	EV_GUI_RButtonUp,
 	EV_GUI_RButtonDblClick,
-	EV_GUI_WheelDown,		// data3: shift/ctrl/alt
-	EV_GUI_WheelUp			// data3: shift/ctrl/alt
+	EV_GUI_WheelUp,			// data3: shift/ctrl/alt
+	EV_GUI_WheelDown,		// "
+	EV_GUI_WheelRight,		// "
+	EV_GUI_WheelLeft,		// "
 };
 
 enum GUIKeyModifiers
diff --git a/src/doomdef.h b/src/doomdef.h
index bc48a78a0..8fa587a11 100644
--- a/src/doomdef.h
+++ b/src/doomdef.h
@@ -167,8 +167,10 @@ enum ESkillLevels
 
 #define KEY_MWHEELUP			0x198
 #define KEY_MWHEELDOWN			0x199
+#define KEY_MWHEELRIGHT			0x19A
+#define KEY_MWHEELLEFT			0x19B
 
-#define NUM_KEYS				0x19A
+#define NUM_KEYS				0x19C
 
 #define JOYAXIS_NONE			0
 #define JOYAXIS_YAW				1
diff --git a/src/win32/i_input.h b/src/win32/i_input.h
index b474ffb69..79502bba4 100644
--- a/src/win32/i_input.h
+++ b/src/win32/i_input.h
@@ -76,11 +76,11 @@ public:
 
 protected:
 	void PostMouseMove(int x, int y);
-	void WheelMoved(int wheelmove);
+	void WheelMoved(int axis, int wheelmove);
 	void PostButtonEvent(int button, bool down);
 	void ClearButtonState();
 
-	int WheelMove;
+	int WheelMove[2];
 	int LastX, LastY;	// for m_filter
 	WORD ButtonState;	// bit mask of current button states (1=down, 0=up)
 };
diff --git a/src/win32/i_mouse.cpp b/src/win32/i_mouse.cpp
index 410c9a411..611784a5a 100644
--- a/src/win32/i_mouse.cpp
+++ b/src/win32/i_mouse.cpp
@@ -32,6 +32,12 @@
 #define GET_XBUTTON_WPARAM(wParam) (HIWORD(wParam))
 #endif
 
+// Only present in Vista SDK, and it probably isn't available with w32api,
+// either.
+#ifndef WM_MOUSEHWHEEL
+#define WM_MOUSEHWHEEL	0x20e
+#endif
+
 // TYPES -------------------------------------------------------------------
 
 class FRawMouse : public FMouse
@@ -266,7 +272,8 @@ FMouse::FMouse()
 {
 	LastX = LastY = 0;
 	ButtonState = 0;
-	WheelMove = 0;
+	WheelMove[0] = 0;
+	WheelMove[1] = 0;
 }
 
 //==========================================================================
@@ -308,18 +315,20 @@ void FMouse::PostMouseMove(int x, int y)
 // Generates events for a wheel move. Events are generated for every
 // WHEEL_DELTA units that the wheel has moved. In normal mode, each move
 // generates both a key down and a key up event. In GUI mode, only one
-// event is generated for each unit of movement.
+// event is generated for each unit of movement. Axis can be 0 for up/down
+// or 1 for left/right.
 //
 //==========================================================================
 
-void FMouse::WheelMoved(int wheelmove)
+void FMouse::WheelMoved(int axis, int wheelmove)
 {
+	assert(axis == 0 || axis == 1);
 	event_t ev = { 0 };
 	int dir;
 
-	WheelMove += wheelmove;
+	WheelMove[axis] += wheelmove;
 
-	if (WheelMove < 0)
+	if (WheelMove[axis] < 0)
 	{
 		dir = WHEEL_DELTA;
 		ev.data1 = KEY_MWHEELDOWN;
@@ -329,30 +338,31 @@ void FMouse::WheelMoved(int wheelmove)
 		dir = -WHEEL_DELTA;
 		ev.data1 = KEY_MWHEELUP;
 	}
+	ev.data1 += axis * 2;
 
 	if (!GUICapture)
 	{
-		while (abs(WheelMove) >= WHEEL_DELTA)
+		while (abs(WheelMove[axis]) >= WHEEL_DELTA)
 		{
 			ev.type = EV_KeyDown;
 			D_PostEvent(&ev);
 			ev.type = EV_KeyUp;
 			D_PostEvent(&ev);
-			WheelMove += dir;
+			WheelMove[axis] += dir;
 		}
 	}
 	else
 	{
 		ev.type = EV_GUI_Event;
-		ev.subtype = (WheelMove < 0) ? EV_GUI_WheelDown : EV_GUI_WheelUp;
+		ev.subtype = ev.data1 - KEY_MWHEELUP + EV_GUI_WheelUp;
 		if (GetKeyState(VK_SHIFT) & 0x8000)		ev.data3 |= GKM_SHIFT;
 		if (GetKeyState(VK_CONTROL) & 0x8000)	ev.data3 |= GKM_CTRL;
 		if (GetKeyState(VK_MENU) & 0x8000)		ev.data3 |= GKM_ALT;
 		ev.data1 = 0;
-		while (abs(WheelMove) >= WHEEL_DELTA)
+		while (abs(WheelMove[axis]) >= WHEEL_DELTA)
 		{
 			D_PostEvent(&ev);
-			WheelMove += dir;
+			WheelMove[axis] += dir;
 		}
 	}
 }
@@ -415,7 +425,8 @@ void FMouse::ClearButtonState()
 		ButtonState = 0;
 	}
 	// Reset mouse wheel accumulation to 0.
-	WheelMove = 0;
+	WheelMove[0] = 0;
+	WheelMove[1] = 0;
 }
 
 //==========================================================================
@@ -597,7 +608,11 @@ bool FRawMouse::WndProcHook(HWND hWnd, UINT message, WPARAM wParam, LPARAM lPara
 			}
 			if (raw->data.mouse.usButtonFlags & RI_MOUSE_WHEEL)
 			{
-				WheelMoved((SHORT)raw->data.mouse.usButtonData);
+				WheelMoved(0, (SHORT)raw->data.mouse.usButtonData);
+			}
+			else if (raw->data.mouse.usButtonFlags & 0x800)	// horizontal mouse wheel
+			{
+				WheelMoved(1, (SHORT)raw->data.mouse.usButtonData);
 			}
 			PostMouseMove(m_noprescale ? raw->data.mouse.lLastX : raw->data.mouse.lLastX<<2,
 				-raw->data.mouse.lLastY);
@@ -794,7 +809,7 @@ void FDInputMouse::ProcessInput()
 		}
 		else if (od.dwOfs == (DWORD)DIMOFS_Z)
 		{
-			WheelMoved(od.dwData);
+			WheelMoved(0, od.dwData);
 		}
 		else if (od.dwOfs >= (DWORD)DIMOFS_BUTTON0 && od.dwOfs <= (DWORD)DIMOFS_BUTTON7)
 		{
@@ -963,7 +978,12 @@ bool FWin32Mouse::WndProcHook(HWND hWnd, UINT message, WPARAM wParam, LPARAM lPa
 	}
 	else if (message == WM_MOUSEWHEEL)
 	{
-		WheelMoved((SHORT)HIWORD(wParam));
+		WheelMoved(0, (SHORT)HIWORD(wParam));
+		return true;
+	}
+	else if (message == WM_MOUSEHWHEEL)
+	{
+		WheelMoved(1, (SHORT)HIWORD(wParam));
 		return true;
 	}
 	else if (message >= WM_LBUTTONDOWN && message <= WM_MBUTTONUP)