The purpose of the study was to assess the accuracy, using electronic portal imaging, of daily set-ups, in patients undergoing radiotherapy for prostate carcinoma. We used a scanning liquid ion chamber to assess the accuracy of set-ups in 25 consecutive patients undergoing a 6 1/2 week course of radiotherapy to the prostate. Electronic images (EPIs) were collected during 33 treatments to each of four ports. The positions of anatomical structures on the EPIs were compared with the same structures seen on digitally reconstructed radiographs (DRRs) made after CT simulation before beginning radiotherapy. Displacements of the EPIs compared with the DRRs were computer-calculated in millimetres in lateral, longitudinal and rotational directions for each port. 11 patients had ports moved because of discrepancies between the EPIs and the DRRs; eight required moves in the first five treatments to correct systematic (simulator) errors. In the right-left and anterior-posterior directions, nearly 95% of the EPIs were within 5 mm of the simulated port position. In the superior-inferior direction, 98% of the ports were within 5 mm of the simulated port position. Two patients had in-plane rotational errors on the lateral ports (8 degrees and 10 degrees respectively). It was concluded that daily electronic imaging is an effective technique for assessing the accuracy of set-ups in prostate radiotherapy.