BACKGROUND Anti-CD(25) monoclonal antibodies (Mabs) have been evaluated for the treatment of steroid-refractory acute graft-versus-host disease (GVHD) in patients undergoing hematopoietic stem cell transplantation (HSCT) mainly with matched donors for years, but there is little attention concerning patients with unmanipulated human leukocyte antigen (HLA)-mismatched/haploidentical transplantations. We investigated the efficacy and safety of the chimeric Mab, basiliximab, to treat steroid-refractory acute GVHD after unmanipulated mismatched/haploidentical HSCT. METHODS Fifty-three patients who developed steroid-refractory acute GVHD between July 2005 and July 2009 were treated at our institute with basiliximab. No prisoners were used in this study. RESULTS Forty-six among 53 patients responded, including 37 complete remissions at a median response of 6 days from Mab initiation. There were 29 episodes of viral reactivations, 25 bacterial infections, and 11 probable fungal infections. Thirty-four out of 49 patients who could be evaluated developed chronic GVHD. Twenty-eight of 53 subjects (52.8%) were alive at a median follow-up of 16 months (range, 2-57) posttransplantation. The Kaplan-Meier probability of a 3-year event-free survival was 47.7%. The causes of death were infection alone (n = 15), progressive GVHD with infection (n = 3), relapse (n = 3), and other etiologies (n = 4). CONCLUSIONS These data suggested that basiliximab was effective to treat steroid-refractory acute GVHD after unmanipulated HLA-mismatched/haploidentical stem cell transplantation.