Among 28 children under 4 years of age hospitalized with congenital aniridia, Wilms' tumor subsequently developed in six and one had the neoplasm on admission. Five of these seven patients have not, to our knowledge, been reported previously, bringing to 22 the total number of cases known with this association. Although one child in our series had familial aniridia, all other cases of aniridia with Wilms' tumor have been sporadic, suggesting that the two disorders originate by common or concurrent gene mutation or by environmental effects on embryogenesis. The aniridia-Wilms' tumor complex includes an array of nonocular anomalies, providing further evidence that the neoplasm is but one manifestation of a distinctive syndrome.