We evaluated the impact of busulfan dose intensity in patients undergoing reduced toxicity/intensity conditioning allogeneic transplantation in a multicenter retrospective study of 112 consecutive patients. Seventy-five patients were conditioned with busulfan (0.8 mg/kg/dose IV × 8 doses), fludarabine (30 mg/m2/day, days −7 to −3), and 6 mg/kg of ATG [reduced intensity conditioning (RIC) group], while 37 patients received a more-intense conditioning with busulfan (130 mg/m2/day IV, days −6 to −3), fludarabine (40 mg/m2/day, days −6 to −3) and 6 mg/kg of ATG [reduced toxicity conditioning (RTC) group]. At baseline both groups were matched for median age, unrelated donor allografts, and human leukocyte antigen-mismatched allografts. More patients in RIC group had high-risk disease, and higher median comorbidity index. There were no graft rejections. Median time to neutrophil (17 days vs. 15 days; p = 0.003) and platelet engraftment (16 days vs. 11 days; p < 0.001) was significantly longer in the RIC group. RTC group had significantly more bacterial (62.2% vs. 32%; p = 0.004) and fungal infections (13.5% vs. 1.3% p = 0.01). For RIC and RTC groups rates of grades II–IV acute GVHD (34% vs. 40%; p-value = 0.54), and chronic GVHD (45% vs. 57%; p-value = 0.30) were not significantly different. In similar order at 1 year the cumulative-incidence of non-relapse mortality (NRM; 12% vs. 21%; p-value = 0.21) and relapse rates (38% vs. 39%; p = 0.96) were not significantly different. Patients in RIC and RTC groups had similar 1-year overall survival (61% vs. 50%, p = 0.11) and progression-free survival (50% vs. 36%, p-value = 0.39). Our data suggest that the merits of higher busulfan dose intensity in the context of fludarabine/busulfan-based RTC may be offset by higher early morbidity. Copyright © 2011 John Wiley & Sons, Ltd.