The reproducibility of 3-dimensional (3D) conformal therapy in localized prostate cancer was studied in 14 patients, based on retrospective analysis of 196 anteroposterior-posteroanterior (AP/PA) and lateral portal images. The patients were treated supine without rigid immobilization using six isocentric coplanar conformal fields. Three different observers independently compared the portal and simulation images, determining the deviation of each portal film from the corresponding simulation film. No significant deviations were observed in the cephalo-caudal or lateral axes (0 mm median values). However, a systematic median shift of 5 mm (0 to + 10, range) was observed in the anteroposterior direction, presumably as a consequence of a sagging in the treatment couch under the patient's weight. After modification of the treatment couch, no further systematic anteroposterior shifts have been observed. These results demonstrate that the daily setup of conformal prostate irradiation fields can be performed with acceptable reproducibility without the use of special immobilization devices.