Reduced intensity conditioning (RIC) have allowed the application of transplantation to older patients and to patients with underlying medical problems. Between October, 1999, and June, 2003, 61 patients with acute leukemia or chronic myeloid leukemia received allogeneic peripheral blood hematopoietic stem cell transplantation (HSCT) from HLA-identical siblings. Thirty-four were conditioned with myeloablative protocols and twenty-seven with RIC regimens. The patients in the myeloablative group were younger (29 vs. 37 years; p < 0.0003), most of them were transplanted in complete remission (74% vs. 59%; p < 0.03), had a shorter interval between diagnosis and HSCT (12 vs. 21 months; p < 0.02) and a greater proportion belonged to standard-risk prognosis (68% vs. 48%; p < 0.1). The median times to neutrophil, platelet and red blood cell engraftment for the myeloablative and RIC groups were 14 versus 11 days (p < 0.009), 17 versus 9 days (p < 0.0001), and 19 versus 12 days (p < 0.007), respectively. Transfusion requirements were lower in the RIC group. Severe mucositis was present in 32% and 7%, respectively (p < 0.01). The proportion of patients having acute graft versus-disease (GVHD), chronic GVHD, and infections was the same, as well as early and late mortality, disease-free survival, and overall survival. Analyzing all the patients together, three factors significantly influenced overall survival: standard risk patients, complete remission at transplant, and the absence of severe acute GVHD. In conclusion, our data suggest that even in high-risk patients, RIC transplantation seems to be as useful as ablative HSCT.