OBJECTIVE Nocturnal penile tumescence monitoring was compared to penile duplex ultrasonography and pharmaco-infusion cavernosometry in 50 cases of erectile dysfunction. METHODS Nocturnal penile tumescence was evaluated in all patients as normal or abnormal according to standard general criteria. The results were compared to penile duplex ultrasonography parameters (peak systolic velocity, normal greater than 35 cm. per second, and diastolic velocity, normal less than 5 cm. per second), and to the flow rate needed to maintain erection (normal less than 15 ml. per minute) with pharmaco-infusion cavernosometry. RESULTS Of the 50 patients 26 had normal nocturnal penile tumescence, including 25 (96%) with normal penile velocity, 18 (69%) with normal penile diastolic velocity and 22 (85%) with normal flow to maintain erection. On the other hand, 24 men had abnormal nocturnal penile tumescence of whom 7 (29%) had abnormal penile blood flow velocity, 17 (71%) had abnormal diastolic flow velocity and 18 (75%) had high flow rate to maintain erection. CONCLUSIONS Normal nocturnal penile tumescence appears to correlate well with normal systolic blood velocity and cavernosometry but poorly with diastolic blood velocity. On the other hand, a low correlation exists between abnormal nocturnal penile tumescence and abnormal diastolic blood flow or abnormal cavernosometry. Furthermore, no correlation exists between abnormal nocturnal penile tumescence and abnormal systolic blood flow. According to this observation we presume that nocturnal penile tumescence, penile duplex and infusion cavernosometry should be performed to achieve a reasonably accurate diagnosis.