Recent interest has focused on the function of γδ+ T cells in immune responses. However, their role in allogeneic bone marrow transplantation (BMT) remains undefined. We report on a group of 43 leukemia patients who survived for at least 100 days following transplantation using partially HLA-mismatched grafts from related donors that were T cell depleted with the anti-TCRαβ monoclonal antibody T10B9.1A-31 and complement. Ten patients (23.2%) were found to have an increased (≥10%) proportion of γδ+ T cells in the peripheral blood at 60-270 days after BMT. All of these patients remain alive, and 9 (90% of patients with ≥10% γδ+ cells) are free of disease at 2.5 years compared with a disease-free survival probability of 31% among patients with a normal proportion and concentration of γδ+ T cells. No other factor was found to be independently associated with improved survival in these patients. These data suggest a possible association between an increase in the percentage and number of γδ+ T cells and improved disease-free survival following transplantation from a partially mismatched related donor.