A cytogenetically normal man with severe aplastic anemia was treated with granulocyte colonystimulating factor (G-CSF), erythropoietin (EPO), cyclosporin A, anti-thymocyte globulin, and interleukin-6 (IL-6), which resulted in a gradual improvement in his neutrophil count and hemoglobin level. After 2 years of the therapy, monosomy 7 was detected during cytogenetic analysis of his bone marrow, which evolved during a period of 5 months into acute myeloblastic leukemia. An in vitro proliferation assay of cytokine responses showed that leukemic blasts were sensitive only to G-CSF, and not to EPO or IL-6. Although allogeneic bone marrow transplantation from an HLA-matched unrelated donor was carried out in the non-remission stage, the patient died of systemic fungal infection on day 25, without any evidence of hematological engraftment. As long-term use of cytokines and immunomo-suppressants in patients with severe aplastic anemia may induce or hasten the onset of a malignant transformation, careful attention must be paid to clonal evolution. Due to the poor prognosis of secondary myelodysplasia and leukemia, allogeneic bone marrow transplantation for such patients must be carried out early in the course of the disease.