Twelve new cases of childhood leukemia and neurofibromatosis were ascertained and evaluated in conjunction with 17 previously well-documented cases. The ratio of ALL:nonlymphocytic leukemia was 9:20, markedly different from the 4:1 ratio in children without NF. Rarer subtypes predominated: 8 CML and 8 AMML. The peculiar distribution of leukemia by cell type and the number of cases observed in the United States indicate that the risk of childhood leukemia in NF is increased. Two possible variants were noted: NF with "transient leukemia," and multiple skin xanthomas with nonlymphocytic leukemia.