A comparison was made of blood loss under controlled heparinization and that occurring under normal conditions during electroresection of the prostate. Mean blood loss in patients treated with heparin amounted to 429.4 +/- 524.8 ml. and in the control group it was 391.8 +/- 451.4 ml. The difference is not significant. Therefore, a transurethral operation does not require interruption of anticoagulation therapy in cardiovascular patients.