The possibility of statistically significant correlation between blood loss during transurethral prostatectomy (TURP) and weight of the resected tissue and/or duration of the operation was explored in 392 patients. All had been operated on by the same surgeon, and in all cases the peroperative blood loss, weight of resected tissue and length of operation were known. A highly significant correlation was found between blood loss and tissue weight, with an overall mean of 15 ml per gram. The severity of bleeding, however, increased with duration of the operation, from 5.2 ml per minute in the smallest glands (less than 10 g) to 20.3 in the largest (greater than 80 g). Speedier surgery is therefore one possibility of reducing blood loss in TURP. In most of the cases with heavy peroperative bleeding, the operating time exceeded one hour. Surgeons should therefore try not to undertake resections that cannot be completed within 60 minutes. If the surgeon is experienced and the estimated prostatic weight less than 30 grams, preoperative crossmatching of blood is not usually necessary.