Cranioplasties using intraoperatively modeled prostheses may fail to create harmonic contours with long-term stability. In contrast, preoperative modeling would allow more sophisticated planning of the contour and better preparation of the implant material, if a sufficiently precise model of the defect-site was available. In this respect, computer aided design and manufacturing (CAD/CAM)-techniques based on helical computed tomography (CT) data are successfully used for the prefabrication of prostheses: An individual computer-based 3-dimensional model of the bony defect is generated after acquisition, transfer and evaluation of the CT data; from this freeform surfaces geometry an individual and "idealized" prosthesis-geometry is derived and fabricated by a numerically controlled milling machine using modern industrial CAD/CAM-systems and design software. The margins of this prosthesis-geometry are generated by the borders of the defect and the surface by considering the non-affected neighbouring contours. Cranioplasties in cases of large postsurgical skull defects are presented as the first clinical applications of this new method, which also allows the use of titanium and fabrication of integrated fixation-devices.