Postoperative bleeding in patients who regularly ingest acetylsalicylic acid (ASA) has been reported after several types of surgery. However, data on the influence of ASA on the risk of haemorrhage from transurethral prostatectomy (TUR-P) have been conflicting. We have studied retrospectively the unselected clinical records of all patients undergoing TUR-P in the Department of Urology at Hvidovre Hospital (during 1992-1994) with special focus on the use of ASA and non-steroidal anti-inflammatory drugs (NSAIDs). In total, 457 records were examined: 99 patients on ASA/NSAID received 42 units of blood, while 358 patients free from such medication received 68 units of blood, a significantly smaller amount (p = 0.0390). We conclude that ASA and NSAIDs increase the risk of bleeding during and after TUR-P, and we recommend the withdrawal of these drugs for one week before TUR-P.