Yasculogenic impotence is produced by abnormalities of vascular blood supply or drainage, and is the most common cause of various organic impotences. An increasing awareness of vascular causes of impotence has resulted from development of various diagnostic tests, but precise measurement of penile blood flow is difficult. Radionuclide penogram has been introduced recently to diagnose vasculogenic impotence. Forty-one impotent patients and 12 normal men were studied with radionuclide erection penogram using Tc-99m pertechnetate and an intracavernous injection of papaverine. We defined arteriogenic impotence as arterial index less than 0.66, and venogenic impotence as venous index greater than 0.09. By this criteria, the false positive ratio in normal men was 17%, and the false negative ratio in radically cystectomized patients was 0%. Side effects were small purpura of the penile shaft and dull pain during injection of papaverine. The radionuclide erection penogram was noninvasive and gave a dynamic evaluation of the arterial supply, venous drainage, and blood flow in the corporeal bodies. This method should be considered as a valuable adjunct to evaluate patients with vasculogenic impotence.