A fundamental abnormality in acute myeloid leukemia is a block in cell differentiation with resultant accumulation of immature leukocytes. This abnormality can be studied in continuously growing leukemic cell lines that differentiate with simple chemical signals. Surface antigenic modulation occurring with cell differentiation can be monitored by specific antisera. These antisera have great potential as diagnostic and therapeutic reagents. More than 90% of patients with acute lymphoblastic leukemia and more than 70% of patients with acute myeloid leukemia can achieve remission of disease with aggressive multiagent chemotherapy. Long-term, disease-free survival is obtainable in about one half of patients with acute lymphoblastic leukemia but in less than 15% of patients with acute myeloid leukemia. The future directions of research for achieving cure of acute leukemia seem to be well defined.