Preoperative planning of orthognathic surgery is usually based on plaster cast models. Those are integrated in an articulator, simulating the movement of the jaws. We present a new procedure to combine the individual jaw anatomy by stereolithographical models with dental plaster casts, which are more accurately reproducing dental and bony anatomy. For image fusion the patient and the individual plaster casts are scanned by computed tomography, the occlusal planes being covered by a splint with radioopaque makers. After threshold segmentation of both, the plaster cast and the patients skull images are fused. The plaster cast volume is then subtracted from the patients upper and lower jaw, thus sparing out the place for later fusion of stereolithographic model and plaster cast.