A 64-yr-old female developed acute myeloblastic leukemia following radiation and drug therapy for colon carcinoma. The patient was heterozygous for glucose-6-phosphate dehydrogenase (G-6-PD) and displayed types A and B isoenzymes in nonhematopoietic tissue. In contrast, only type B G-6-PD was observed in peripheral blood white cells. In addition, a karyotypic abnormality was found in peripheral blood and marrow cells but not in skin fibroblasts. The data are consistent with a clonal origin of this leukemia.