Three cases of myeloid leukemia (acute myeloblastic leukemia, chronic myelocytic leukemia, erythroleukemia) were observed among a population of patients with Hodgkin's disease. Chromosome analysis of bone marrow cells showed abnormalities in each case similar to those usually described in the respective types of leukemia. There were no chromosomal abnormalities in bone marrow cells from 28 patients with Hodgkin's disease without leukemia. These findings indicate that the two disorders were separate entities. Other than Hodgkin's disease, radiation therapy was the only common factor in these patients. Five additional patients with coexistent Hodgkin's disease and myeloid leukemia were reported in the literature; all but one of these also had previous irradiation. The median latent period between first irradiation and appearance of myeloid leukemia in the combined series was 7 years. These cases probably represent radiation-induced leukemia. That such cases have occurred predominantly in recent years is consistent with the recent use of higher doses of radiation and with the longer survival of these patients. However, the relatively small risk of radiation-induced leukemia is a reasonable price to pay for improvement in the control of Hodgkin's disease by modern irradiation techniques.