In this study 751 portal images of 18 different patients were analyzed. The analysis of each portal image gives a projection of the 3D set-up deviation in the plane of the imager. The set-up deviation of the patient in all three patient directions: lateral, ventrodorsal and craniocaudal can be determined from at least two portal images of beams with diffe-rent gantry angles by means of a coordinate transformation as described by Siddon7. The accuracy of the calculated set-up deviation in the patient directions depends on the difference in gantry angles. In theory it should be possible to determine the set-up deviation from portal images of oblique beams with the same accuracy as from AP and lateral beams if the difference in gantry angle is 90 degrees. In the appendix A a more detailed description of the analysis is given.