A 22-year-old man presented with mediastinal lymphoma and lymphoblastic leukemia. The leukemic cells in his blood and marrow did not stain positively with Sudan black B, peroxidase, chloroacetate esterase, or nonspecific esterase, and were considered as either lymphoblasts or stem cells. One year after the initial presentation, acute myelomonocytic leukemia developed and he died. The leukemic cells possessed the morphologic and cytochemical characteristics of both monocytes and granulocytes. This case illustrates the close ontogenetic relationship between the monocytes and the granulocytes. It also demonstrates that our present concept of acute myelomonocytic leukemia should be broadened to include cases in which the leukemic cells possess the morphologic and cytochemical characteristics of both the monocytes and the granulocytes.