Here we study the magnitude of prostate motion during the delivery of a radiotherapy fraction. These motions have clinical consequences for on-line position verification and the choice of margins around the target volume. We studied the motion of the prostate for 10 patients during 251 radiotherapy treatment fractions by assessing the position of implanted gold markers. Gold markers of 1 mm diameter and 5 mm length were implanted in the prostate before the start of the radiotherapy. We obtained movies during each fraction using an a-Si flat-panel imager. The markers could be detected in separate frames using a marker extraction kernel. Marker displacements as large as 9.5 mm were detected in one fraction. The motion of the prostate is greatest in the caudal-cranial and the anterior-posterior directions. Within a time window of 2 to 3 min, deviations from the initial marker position, averaged over all patients, are 0.3 +/- 0.5 mm and -0.4 +/- 0.7 mm in the anterior-posterior and caudal-cranial directions, respectively. It appeared that on average, the intrafraction prostate motions did not result in margins larger than 1 mm, provided that the position verification is performed at time intervals of 2 to 3 min. Only for some patients performing more frequent position verification or adding extra margins of 2 to 3 mm is required to account for intrafraction prostate motions.