Several congenital urological anomalies have been shown to be present in monozygotic twins and to have familial tendencies. Most of these disorders are associated with maldevelopment of the mesonephric (wolffian) duct and their genetic predisposition suggests an interaction of many genes of small effect. The etiology of type I posterior urethral valves is unknown. Some authors postulate that these valves result from anomalous development of the mesonephric duct, while others believe that they originate from the urogenital sinus. There are only a few reports in the literature on familial posterior urethral valves. We report on 2 more pairs of monozygotic twins and suggest that posterior urethral valves also may have polygenetic factors similar to other anomalies of the mesonephric duct. Based on this observation, we propose that posterior urethral valves likely arise from maldevelopment of the mesonephric duct.