We are reporting on 7 families with both unilateral and bilateral renal agenesis (or severe dysplasia). This condition, termed hereditary renal adysplasia (HRA) [Buchta et al, 1973], is an autosomal dominant trait with incomplete penetrance and variable expression. Review of the literature on familial renal agenesis suggests that HRA is more common than previously supposed and may account for most recurrences of bilateral renal agenesis (BRA), even when the parents are normal. There are no consistent phenotypic differences between sporadic and familial renal agenesis. Associated non-urogenital anomalies, although more frequent in sporadic cases, have been reported in HRA. Use of several approaches, including the Weinberg Proband Method, segregation analysis, and empiric risk estimation, leads to the conclusion that autosomal dominant inheritance is the most likely pattern of transmission for most cases of renal agenesis. Penetrance is between 50% and 90%. Ultrasound study of the kidneys of parents, sibs, and other relatives is recommended in all families in which there is an individual with unilateral or bilateral renal agenesis. The empiric risk for recurrence of BRA in sibs has been estimated at 3.5% [Carter, et al, 1979] but in the offspring of affected or obligate heterozygotes for HRA, the empiric risk of bilateral severe renal adysplasia is 15-20%.