• The finding of three cases of acute leukemia and one of lymphosarcoma in a family of 12 siblings prompted this study of possible causation. Since no environmental cause could be identified, the likelihood of a genetic cause was investigated by constructing a pedigree and attempting to determine the causes of death among the many relatives of the four patients. The possibility that a recessive gene was involved was especially considered because of the finding of evidences of consanguinity, including the fact that the parents were first cousins.