Peyronie's disease is an ill-defined condition that often leads to severe penile deformity and sometimes erectile dysfunction. Penile Doppler studies indicate veno-occlusive dysfunction as the principal cause of poor rigidity in Peyronie's disease patients. Diabetes mellitus is also a known cause of impotence and its prevalence tended to be higher in patients with Peyronie's disease. We evaluated 143 patients with Peyronie's disease, also 92 impotent men (37 diabetic, 55 nondiabetic) as control group. Penile vascular studies were performed on each group. Diabetes mellitus was also investigated in patients with Peyronie's disease. Veno-occlusive dysfunction was found as the main cause of impotence in both groups (81.1% in diabetic, 89% in nondiabetic control group and 88.8% in Peyronie's disease patients). In our study group the prevalence of diabetes mellitus tended to be higher than in the control population but it did not alter the ratio of veno-occlusive dysfunction and impotence in Peyronie's disease patients. We believe there is a close relationship between diabetes mellitus and Peyronie's disease, considering our high incidence. We also conclude that diabetes mellitus is not the main cause of impotence in patients with Peyronie's disease.