A double-blind prospective study was performed on 34 patients undergoing transurethral resection of the prostate (TURP). Seventeen patients, selected at random by computer, were given 5,000 units of subcutaneous heparin three hours preoperatively and every twelve hours thereafter for forty-eight hours. The other 17 received normal saline injections in a similar manner. All patients had pre- and postoperative lung scans. The blood loss was measured during and for seventy-two hours after surgery. There was no statistically significant difference in the bleeding in comparing the two groups. Two patients from the heparin group were diagnosed to have pulmonary emboli by lung scan preoperatively and showed no change postoperatively. In 1 patient from the control group an asymptomatic pulmonary embolism developed in the postoperative period. The use of subcutaneous heparin in TURP is safe and is recommended for patients with an increased risk of pulmonary embolism.