The early toxicity, incidence of graft-versus-host disease (GVHD) and long-term follow-up were evaluated in two children with Down syndrome (DS) treated for acute lymphoblastic leukemia (ALL) in second complete remission by HLA-matched sibling allogeneic bone marrow transplantation (BMT). Preparative conditioning therapy consisted of cytosine arabinoside (Ara-C) and fractionated total body irradiation (F-TBI) and GVHD prophylaxis of cyclosporin A. The conditioning regimen was well tolerated, the only acute complication being mild mucositis. Engraftment (polymorphonuclear cells >500/microliter) was documented by day +17 in both patients. One child remains in continuous complete remission, without medical problems, 60 months after BMT. The second patient died from complications associated with chronic GVHD 21 months following BMT. Ara-C and F-TBI is a well-tolerated preparative regimen for children with DS undergoing allogeneic BMT.