Renal multicystic dysplasia is commonly regarded as a sporadic anomaly, although several studies of renal agenesis have shown the occasional occurrence of multicystic dysplasia in relatives of propositi with bilateral agenesis. We report the occurrence of unilateral multicystic dysplasia in an infant whose mother and maternal aunt had unilateral renal agenesis. The aunt's daughter, a first cousin of the proposita, had unilateral megaureter and hydronephrosis secondary to obstruction at the ureteropelvic junction. These observations suggest that nonsyndromal multicystic renal dysplasia can occur as part of the spectrum of hereditary renal adysplasia, with what appears to be autosomal dominant inheritance, and that the primary abnormality may reside in the ureter, with secondary renal maldevelopment.