A prospective randomised study of subcutaneous heparin compared with no heparin was performed in 60 patients undergoing transurethral resection of the prostate. Operative blood loss in the irrigation fluid was measured using a haemoglobinometer. Blood loss correlated with the amount of prostate resected and the duration of the procedure. Subcutaneous heparin had no significant effect on the measured blood loss in transurethral resection of the prostate and can be used as a safe means of prophylaxis against thrombo-embolic complications.