The risk of cancer was examined among family members of 9 patients with Fanconi's anemia. Seven cancers of diverse types were observed, as compared with the 10.4 expected (P greater than 0.05). In addition, the study of relatives of 60 patients with acquired aplastic anemia showed no unusual distribution of cancers by site and age at diagnosis. In particular, no relative in either series had acute leukemia. Methodologic issues complicate the interpretation of several published reports of excess cancers among heterozygotes of Fanconi's anemia, ataxia-telangiectasia, and xeroderma pigmentosum.