Objectives. To determine the efficacy of tamsulosin in preventing acute urinary retention following early catheter removal after radical retropubic prostatectomy. Methods. Between February 2000 and October 2000, cystography was performed on postoperative day 7 after radical retropubic prostatectomy by a single surgeon (group 1). Between September 2001 and August 2002, cystography was performed on postoperative day 8 after radical retropubic prostatectomy by the same surgeon (group 2). The protocol for performing cystography and assessment of extravasation was similar for both groups. Tamsulosin 0.4 mg was administered 3 days before and 4 days after cystography for all men in group 2. Results. Of 179 cystograms in group 1, 135 (75%) revealed no extravasation, and the catheters were removed in 130 of these cases. Of 246 cystograms in group 2, 230 (93.5%) revealed no extravasation, and the catheters were removed in 229 of these cases. A significantly greater proportion of men in group 2 had no extravasation (P = 0.0007). The incidence of acute urinary retention in groups 1 and 2 was 10% and 2.6%, respectively (P = 0.0018). The incidence of anastomotic stricture was not significantly different between the two groups. Conclusions. Our data strongly suggest that tamsulosin significantly reduces the risk of acute urinary retention after attempts at early catheter removal following radical retropubic prostatectomy. Therefore, we recommend administering a 7-day course of tamsulosin therapy when attempting to remove the urinary catheter before postoperative day 8.