The Le Fort I osteotomy is one of the most frequently used procedures in orthognathic surgery. The preoperative planning of a Le Fort I osteotomy usually involves a two-dimensional lateral radiograph. On the lateral radiograph, the movement of the maxilla in the sagittal and vertical planes can be simulated, and the position of the osteotomy line in relation to the occlusion can be determined. However, the lateral radiograph consists of two dimensions, whereas, during surgery, three dimensions have to be considered. The contour of the lateral surface of the maxilla and its individual regional variations are not reflected on the lateral radiograph. Therefore, this third dimension's influence on treatment planning is investigated. Discrepancies of the position of planned osteoto my cuts are described after the transfer from two to three dimensions. Discrepancies of the positions of reference points and lines are up to 3.3 mm. Therefore, it is not possible to transfer reference points and reference lines from the lateral radiograph to the maxilla nor to move the mobilized maxilla along the planned osteotomy lines in a precise manner. It is possible to indicate the general direction of the osteotomy lines. Results of this study show the necessity for the precise control of three-dimensional positioning of the maxilla during treatment planning, cast surgery, and actual surgery.