In 657 impotent patients in whom erection was induced by intracavernous injection of papaverine, we evaluated changes in the pulsation, diameter, and blood flow velocity of the penile arteries by duplex sonography and pulsed Doppler spectrum analysis. An additional 18 patients who were proved to have good erection by nocturnal tumescence testing were used as controls. All patients were studied before achieving full erection after papaverine injection. In the flaccid state, the inner diameter of the deep penile arteries averaged 0.51 mm, and blood flow velocity was rarely measurable. The normal arterial reaction to papaverine was an initial increase in the inner diameter of the scanned segment of the cavernous artery with good concentric pulsations and a mean peak blood flow velocity of 34.8 cm per second and a mean diameter of 0.89 mm. With this technique, we assessed the functional capability of each deep and dorsal artery. Arterial and erectile response allowed indirect evaluation of the venous system. Of the 657 patients, 48 had neurogenic or psychogenic impotence or both. In 97, impotence could be ascribed to venogenic causes, in 210 to arteriogenic causes, and to mixed arteriogenic and venogenic causes in 282 patients. The exact cause of impotence in the remaining 20 patients could not be determined. Statistically, patients who were older or had diabetes mellitus or coronary artery disease tended to have smaller penile arteries and lower peak blood flow velocity after papaverine injection.