Wilms tumor affects approximately one child per 10,000 worldwide before the age of 15 years. Incidence rates appear to be slightly elevated for U.S. and African Blacks in comparison to Whites, but are only half as great among Asians. Several case-control studies have suggested that paternal occupational or maternal hormonal exposures during pregnancy may increase the risk of Wilms tumor, but small numbers of subjects and inconsistencies in the patterns of exposures do not permit firm conclusions to be drawn. It is unlikely that such environmental exposures play a major role in the etiology of Wilms tumor. The median age-at-onset of Wilms tumor is 38 months in the U.S. National Wilms Tumor Study series, with cases in girls occurring on average 6 months later than in boys. Patients with bilateral tumors, aniridia, cryptorchism/hypospadias, Beck-with-Wiedemann syndrome, or intralobar nephrogenic rests tend to be diagnosed much younger than average (median 17-27 months). Those with familial disease or multicentric tumors have intermediate age-at-onset distributions, while those with perilobar nephrogenic rests are diagnosed at older ages. The epidemiologic features suggest that somatic mosaicism, rather than a germline mutation, may be responsible for some of the bilateral and multicentric cases.