A 76-year-old man who was admitted to the hospital had mixed lymphocytic and myelomonocytic leukemia. A review of the world literature revealed 21 previously reported cases dating from 1906. Most of these patients had initial chronic lymphocytic leukemia with the subsequent development of myeloblastic or monoblastic leukemia. In some patients, development of the second form of leukemia may have been related to prior exposures to ionizing radiation or alkylating agents.