The HLA genotypes of members of 104 families of patients with acute myelogenous leukemia (AML), 89 families of patients with acute lymphocytic leukemia (ALL), and 40 control families were analyzed. The results indicate that the HLA genotype of both ALL and AML leukemic patients is found more frequently than the expected 25% among their siblings; the increase for AML patients is statistically significant. This suggests that an HLA-linked factor(s) conferring susceptibility to leukemia may also be associated with increased gametic survival. The chance of finding an HLA-matched sibling for bone marrow transplantation is increased.