Radioisotope erection penography of 113 consecutive impotent patients (41 with psychogenic and 72 with vasculogenic impotence) and 15 normal potent men were obtained. Twenty minutes after intracavernous injection of 99mtechnetium-pertechnetate 40 mg. papaverine hydrochloride were administered into the corpus cavernosum to induce erection. A gamma camera with a pinhole collimator was used to monitor the radioactivity. Various penogram indexes were calculated from the time activity curve and their usefulness was evaluated. Index A1 was useful to differentiate vasculogenic and psychogenic importance. Indexes V1 and V2 were useful to differentiate arteriogenic and venogenic impotence. The radioisotope erection penogram is a simple, less invasive and valuable screening test in the identification of vasculogenic impotence, and is effective in differentiating arteriogenic and venogenic impotence.