A total of 144 evaluations of cell surface markers and cellular immune functions were carried out in 57 patients undergoing allogeneic bone marrow transplantation for acute leukemia in remission and relapse and for aplastic anemia. The periods tested were pretransplant, and 1–3, 4–6, 7–12 and more than 12 months posttransplant. The determination consisted of lymphocyte counts; lymphocyte surface marking using OKT3, OKT4, and OKT8 antibodies; and determination of adherent cells, lysozymes and antibody dependent cellular cytotoxicity (ADCC) against chicken red blood cells, human red blood cells, and CEM cells. Natural killer cells were determined against K562 target cells. Lymphoblastic responses were tested after stimulation with pokeweed mitogen (PWM), concanavalin-A (Con-A), and phytohemagglutinin (PHA). We found that the progression in the leukemic state (first remission, second remission, and replase), prior to transplantation was paralleled by a decrease in T4 lymphocytes (976/μl ± 462; 411/μl ± 222; 372/μl ± 419; P = .04). There was lack of helper cells and an inverted T4: T8 ratio beyond one year posttransplant independent of graft-versus-host disease status. Lymphocyte functions persisted to be depressed for more than one year. We found a direct correlation of T4 helper cells and an inverse correlation of T8 suppressor cells with lymphoblastic responses to mitogens. It is hoped that the longitudinal evaluations of immune functions after allogeneic bone marrow transplantation, and the characterization of the immune defects seen may lead to better immunorestorative treatments.