A total of 78 patients with acute leukemia [69 acute lymphoblastic leukemia (ALL) and 9 acute myeloblastic leukemia (AML)] survived more than 4 years. In the years between 1958 and 1967 the number of patients with ALL in Argentina who survived 4 years was 1%. This increased to 21% in the years between 1967 and 1972. The major cause of this increase was the initiation in 1967 of protocols of therapy on a national scale through a cooperative group known as GATLA. In AML patients the number of long-term survivals remained unchanged during both periods. A large percent of the long-term survivors had lower WBC, a higher platelet count, and no hepato-or splenomegaly, lymphadenopathy, or hemorrhagic manifestations at diagnosis. However, only for those patients with a WBC under 100,OOO/mms at diagnosis was there a significant prognostic implication ( p < 0.01). Patients with acute lymphoblastic leukemia who survive more than 4 years without relapse have about an 80% chance of long survival. For those who have lived 4 years with one or more relapses, there is only a 17% chance of long survival, and of these only those who develop extramedullary relapse (CNS or testicular) without bone marrow involvement have a chance of long survival. Patients who live 4 years in continuous complete remission have the same chance of very long survival regardless of previous therapy.