Seventy-eight male diabetics with sexual dysfunction were evaluated by a thorough history, general physical, psychological, neurological and urological examinations, routine laboratory tests, and a duplex ultrasound scan with intracavernous injection of prostaglandin E₁ (PGE₁). The mean patient age was 55.9 years, and the average onset of sexual dysfunction was 10.0 years after the diagnosis of diabetes. Sixty-eight patients (87.2%) had moderate or severe cavernous arterial insufficiency. Older patients and those having a longer duration of diabetes had a higher incidence of cavernous arterial insufficiency. Cigarette smoking, hypertension, and alcohol abuse were also related to cavernous arterial insufficiency. There was no significant difference in cavernous arterial insufficiency between the insulin-dependent and the insulin-nondependent groups. There were significant differences of diameters and peak blood flow velocities of cavernous arteries between 78 diabetic impotent patients and 10 controls. These findings strongly suggest that the cavernous arterial insufficiency is closely related to the diabetic impotence. In addition, the prevalence of cavernous arterial insufficiency increases with age, duration of diabetes, cigarette smoking, hypertension and alcohol abuse, but it is not definitely correlated with the type of diabetes management.