Chromosome analysis was on tumor cells and peripheral lymphocytes from three cases with Wilms' tumor (WT) carried out. In one of them WT was associated with aniridia, in the other two cases WT developed without any congenital malformations. Deletion in the short arm of chromosome 11 [del(11)(p13)] was found in the tumor cells such as lymphocytes in the patient with aniridia--WT association. The constitutional karyotype of other two patients proved to be normal. In the tumor cells of one of them a normal karyotype was found, while the other child had hyperdiploid karyotype. The role of chromosome 11p deletion, in respect of molecular biological knowledge in tumor genesis, is discussed. Results reported here suggest that children with aniridia associated with a constitutional deletion at the 11p13 locus have a high risk for developing WT and therefore require careful ultrasound follow up.