diff --git a/src/add-ons/input_server/devices/mouse/MouseInputDevice.cpp b/src/add-ons/input_server/devices/mouse/MouseInputDevice.cpp
index 1a5ac8d904..c19ea62bd7 100644
--- a/src/add-ons/input_server/devices/mouse/MouseInputDevice.cpp
+++ b/src/add-ons/input_server/devices/mouse/MouseInputDevice.cpp
@@ -41,7 +41,6 @@
 #include <Path.h>
 #include <String.h>
 
-#define DEBUG 1
 #if DEBUG
 	#define LOG(text) fputs(text, sLogFile); fflush(sLogFile)
 #else
@@ -385,7 +384,13 @@ MouseInputDevice::DeviceWatcher(void *arg)
 		
 		// TODO: add acceleration computing
 		int32 xdelta = movements.xdelta * dev->settings.accel.speed >> 15;
-		int32 ydelta = movements.ydelta * dev->settings.accel.speed >> 15;
+		int32 ydelta = -movements.ydelta * (dev->settings.accel.speed >> 15);
+
+		snprintf(log, 128, "%s: x: %ld, y: %ld, \n",
+                                dev->device_ref.name, xdelta, ydelta);
+
+                LOG(log);
+
 
 		// TODO: B_MOUSE_DOWN and B_MOUSE_UP messages don't seem
 		// to be generated correctly.