Cytogenetic studies have been carried out on cells derived from two Wilms' tumors in vitro. Both tumors had a diploid chromosome range. One tumor was shown to have a definite stemline; 46,XY,4p+,del(9)(q22),11p-q,11p+, and the other range of variation chiefly involving chromosomes No. 11, 4, 7, and 2; most changes in chromosome No. 11 took the form of deletions of the short arm. High resolution chromosome analysis of peripheral blood lymphocytes of the two patients revealed apparently normal karyotypes. These findings suggest that changes in the short arm of chromosome No. 11 are important in the development of Wilms' tumor in normal individuals. This association is reinforced by the fact that patients with spontaneous aniridia with a 1 in 3 risk of Wilms' tumor have been reported as having a specific 11p13 deletion.