One-hundred-thirty-six patients who underwent marrow transplantation for endstage acute leukemia were studied with respect t o factors that might predict failure t o eradicate or recurrence of leukemia following transplantation. One-hundred-five patients were recipients of allogeneic and 3 1 recipients of syngeneic marrow. Sixty-four patients had acute lymphoblastic leukemia (ALL) and 72 acute nonlymphoblastic leukemia (ANL). The frequency of persistence of leukemia and relapse was 35% for patients with ANL and 44% for those with ALL. sidered simultaneously through proportional hazards modeling showed only the WBC at admission for transplantation t o be significant (P = 0.02). The presence of an enlarged spleen, either at diagnosis or at transplantation, was associated with an increased probability of relapse (P < 0.006) among patients with ANL. In patients with ALL, active central nervous system (CNS) disease at the time of transplantation significantly increased the risk of relapse in the CNS (P < 0.01). Patients with a white blood cell (WBC) count greater than 20,000/mm3 at admission for transplantation had an increased rate of relapse (P < 0.003). Patients whose leukemic cells showed a karyotypic abnormality on cytogenetic analysis had an increased probability of relapse as compared t o those with karyotypically normal leukemic cells (P = 0.04). Following transplantation, the persistence of leukemic cells in the marrow one week after transplantation indicated an increased likelihood of failure to achieve remission and an increased likelihood of subsequent relapse in those who achieved a remission (P < 0.001).When all the patients are considered as one group pretransplant factors con-