436 index patients with major congenital urinary tract malformations, 385 with upper and 51 with lower urinary tract anomalies, were studied. A significant male sex predominance was noted in each group. No apparent correlation was found between parental age, birth order, birth weight and incidence of these malformations. The association with abnormalities outside the urinary tract was more frequent in the upper urinary tract group. In this group, the mean consanguinity index was 283-10(-5) and, among those patients where a family study was made, 16.6% had positive family histories. The incidence of urinary tract malformations was 4.0% for the first degree relatives, and 1.1% for second degree relatives (grandparents only). In this group, the concordance rate was 50% in four monozygotic, and 0% (nil) in three dizygotic, twin pairs. The findings, in agreement with studies of other congenital malformations, suggest at least in part, a multifactorial etiology, particularly in the case of upper urinary tract malformations, depending upon genetic predisposition and environmental factors.