Leukemias of indeterminant lineage. Two biologically and clinically important forms of acute leukemia have been described. Evidence suggests that both undifferentiated acute leukemia and many types of hybrid leukemia arise from relatively fewer differentiated cells than do the more straightforward acute leukemias. Clinical correlations suggest that patients with these leukemias may have a poorer prognosis, although some findings may be associated with an improved prognosis. More data in which multiple techniques are applied to the same leukemic cells are clearly needed. Patients with certain types of hybrid leukemias may be suitable candidates for more aggressive forms of chemotherapy or, possibly, combinations of therapy directed at cells of both lineages. 