A 2-year-old girl with sporadic unilateral Wilms' tumor (WT) not associated with aniridia was found to have, besides other chromosome abnormalities, a t(2;7)(q33;p22) in 6% of her lymphocytes. A comparison with 7 previous WT cases without aniridia in whom diverse chromosomal aberrations were present, reveals a wide heterogeneity and lead us to tentatively classify such changes as causal, secondary, and casual.