The parameters of the functional evaluation of the penile arterial system in sexually active males are based on a minimal number of volunteers and impotent patients of neurogenic origin who are expected to have a normal vascular system. In order to investigate these parameters in 23 sexually active diabetic and nondiabetic males, penile arterial systems were evaluated by color Doppler ultrasonography. Parameters obtained from the cavernous arteries were arterial diameter (pre- and postpapaverine), diameter increase rate and systolic peak blood flow velocity. Systolic peak blood flow Velocities in papaverine-induced erection were 36.75 (+/- 9.99) and 37.50 (+/- 13.18) cm/s for right and left cavernosal arteries, respectively, in nondiabetic 16 men. The mean cavernosal artery diameter changes were 89.23 and 77.93% for right and left cavernosal arteries. Systolic peak blood flow velocities were 24.57 (+/- 7.44) and 25.42 (+/- 9.45) cm/s and diameter increase rates were 78.57 and 37.50% for right and left cavernosal arteries in diabetic sexually active men. Sexually active diabetics have a significantly lower cavernosal artery peak blood flow velocity and diameter increase rate than nondiabetics (p < 0.01). Thus a sub-clinic dysfunction of erection might be introduced in diabetic males. In conclusion, each investigator should determine his own standards on sexually active subjects and on those with different etiologies such as diabetes mellitus, hypertension and hypercholesterolemia, contributing to erectile dysfunction.