Abstract.A webcam together with a pen can replace a mouse as pointing device for many common user interaction tasks. We have implemented an imageprocessing component integrated in a tool that acts as mouse alternative. The image-processing component tracks the head of a pen based on shape and colour information retrieved in a quick, integrated initial pen-calibration phase using Hough transform triggered by a motion detection cycle. The tracked 2D position of the pen-head seen by the webcam is used to smoothly position the mouse cursor. Combined with auto-clicking we can replace mouse-based user interaction. The system tolerates changing lighting conditions, does not need time-consuming camera calibration and works with off-the-shelf webcams. First user experiences show that this technology can partially replace mouse interaction for Repetitive Strain Injury (RSI) patients as well as completely replace mouse interaction within dedicated environments such as presentation booths or simple games.