In a population-based case-control study carried out in the Baltimore, Maryland metropolitan area, family cancer history for 342 chronic lymphocytic leukemia cases diagnosed in 1969-1982 revealed significantly higher risks of leukemia as well as other hematolymphoproliferative neoplasms and breast cancer among their first-degree relatives compared with reported occurrence of these neoplasms in first-degree relatives of 342 matched cancer controls and 342 matched controls without cancer. Siblings of case subjects also had a significant elevation of kidney cancer compared with siblings of controls. Only one of the affected case families (and no control family) included more than one additional member with leukemia. The results suggest a genetic component for leukemia occurrence in several case families, although the majority of cases of chronic lymphocytic leukemia appeared to be sporadic. The similarity of findings between the two comparisons (cases vs. cancer controls and cases vs. controls without cancer) diminishes the likelihood of recall bias as an explanation for the observed excess risks.