Leukemias and lymphomas occurring in a series of families with Wilms' tumor (WT) are described. One surviving case developed a large cell anaplastic Ki-1 lymphoma at age 20 years, and 23 second- and higher degree relatives were affected. In two instances leukemia/lymphoma occurred in the context of Li-Fraumeni syndrome (LFS) and two other families showed striking clusters of unusual and early-onset malignancies. In several cases, children had genitourinary abnormalities of the type associated with the WT1 gene on chromosome 11p13. Some of these families may provide important subjects for study of WT genes in hematologic disease and lymphomas and for investigation of interaction between different tumor-suppressor genes, e.g., WT1 and other candidate WT genes, and p53.