This is a study of the occurrence of Wilms' tumor and associated anomalies in all incident cases in the Greater Delaware Valley by race. The average annual incidence of Wilms' tumor in this population of 2 million children is significantly higher among nonwhites than whites. A significantly larger proportion of black cases has a Wilms' tumor-associated condition including aniridia, genito-urinary anomalies, the Beckwith-Wiedemann Syndrome, and hemihypertrophy. For very young patients, there was a greater tendency for blacks to have bilateral tumors or a tumor-associated anomaly, features characteristic of the hereditary form of Wilms' tumor. The excess risk of Wilms' tumor among blacks may be a result of a higher proportion having a hereditary predisposition or more common exposure to agents capable of inducing germinal mutations.