Acute leukemia frequently has been described as a late complication of chemotherapy with alkylating agents in patients treated for multiple myeloma. However, the simultaneous occurrence of multiple myeloma and acute leukemia in the same patient, without previous exposure to chemotherapy, is a rare association. We describe a case of concomitant involvement by multiple myeloma and acute monocytic leukemia. To our knowledge, only 9 such cases have been reported in the literature to date. We discuss the criteria used in diagnosing the 2 separate diseases and the possible mechanisms responsible for this occurrence.