We have analyzed 1448 patients with acquired aplastic anemia, grafted between 2005 and 2009, and compared outcome of identical sibling (n=940) vs unrelated donor (n=508) transplants. When compared to the latter, sibling transplants were less likely to be performed beyond 180 days from diagnosis (39%vs 85%), to have a cytomegalovirus negative donor /recipient status (15% vs 23%), to receive anti-thymocyte globulin in the conditioning (52% vs 61%), and received more frequently marrow as a stem cell source (60% vs 52%). Unrelated donor grafts had significantly more acute grade II-IV (25% vs 13%) and significantly more chronic graft versus host disease (26% vs 14%). In multivariate analysis the risk of death of unrelated donor grafts was higher, but not significantly, compared to a sibling donor (p=0.16). The strongest negative predictor of survival was the use of peripheral blood as a stem cell source (p<0.00001), followed by an interval diagnosis-transplant >180 days (p=0.0005), patient age >20 years (p=0.0005), no anti-thymocyte globulin in the conditioning (p=0.003); and donor/recipient cytomegalovirus serostatus, other than negative/negative ( p=0.04). In conclusion, in multivariate analysis, the outcome of unrelated donor transplants for acquired aplastic anemia, is currently not statistically inferior when compared to sibling transplants, although patients are at greater risk of acute and chronic graft versus host disease . The use of peripheral blood grafts remains the strongest negative predictor of survival.