A 35-year-old male presented with chronic myelomonocytic leukemia (CMMoL) after 6.5 years of alkylating agent therapy for IgG-kappa type multiple myeloma. The total dose of melphalan was 0.648 g. CMMoL was stable with weekly injection of alpha-interferon for one year. Thereafter, monocytosis and thrombocytopenia aggravated, and the patient died of disseminating intravascular coagulation. Prolonged drug therapy can induce CMMoL, as well as other myelodysplastic syndromes.