A patient with acute mixed myelocytic and lymphocytic leukemia is reported. The patient showed two populations of malignant myeloid and lymphoid cells with predominance of myeloid lineage. According to the present knowledge, it is hypothesized that its origin is at a pluripotent transformed stem cell which has the capability of differentiating through myeloid and lymphoid lineages. Its maturation is arrested at certain level, thus raising two monoclonal populations simultaneously in the same patient. The treatment should be combined with drugs used in acute myeloblastic and lymphoblastic leukemia. The response to chemotherapy is generally poor.