We report the presence of an extra chromosomal element in a family with Wilms' tumor (WT). This family has three children, two of whom were affected. One son, the proband, had bilateral and one daughter had unilateral WT. The first child, the father, and the mother did not have WT. The son with bilateral WT had a ring chromosome (R) both in the lymphocytes as well as in the kidney tissue. The size of the ring varied considerably from cell to cell. The daughter with unilateral WT had an abnormal clone containing a small chromosomal ring (r) in phytohemagglutinin (PHA)-stimulated and Epstein-Barr virus (EBV)-transformed lymphocytes. The mother had a karyotype similar to that of the daughter with WT. We hypothesize that the proband's ring chromosome could be the amplified form of the r inherited from the mother. Chromosome 11 was cytogenetically normal in all the cells examined of the affected children and the unaffected mother. In situ hybridization with a centromere-specific DNA cocktail indicated dispersed centromeric DNA both in r and R.