We measured penile blood flow during physiological erection using the Xenon-133 clearance method in order to diagnose the exact cause of impotence. In 16 patients, in whom complete erection was obtained after visual sexual stimulation, the mean penile blood flow during erection was apparently higher than that in the flaccid state. On the contrary, in 6 patients whose reaction to visual stimulation was poor, penile blood flow did not change before and after stimulation. We considered that increased arterial flow was indispensable, but complete venous outlet obstruction was not necessary to produce a rigid erection. In addition, in the flaccid state the mean penile blood flow in the poor response group was apparently higher than that in the response group. This means that some patients who fail to obtain sufficient erection after visual sexual stimulation have increased venous outflow from the corpus cavernosum in the flaccid state. Based on these findings, the main cause of erectile dysfunction in such cases is probably attributed to disorders of the venous outlet system in the cavernous body.