Obesity is an important public health problem that may influence the outcomes of hematopoietic cell transplantation (HCT). We studied 898 children and adults receiving first-time allogeneic hematopoietic stem cell transplants between 2004 and 2012. Pre-transplant body mass index (BMI) was classified as underweight, normal weight, overweight, or obese using the WHO classification, or age-adjusted BMI percentiles for children. The study population was predominantly Caucasian, and the median age was 51 years (5 months – 73 years). The cumulative 3-year incidence of non-relapse mortality (NRM) in underweight, normal weight, overweight, and obese patients was 20%, 19%, 20%, and 33%, respectively. Major causes of NRM were acute and chronic graft-versus-host disease (GVHD). The corresponding incidence of relapse was 30%, 41%, 37%, and 30%, respectively. Three-year overall survival was 59%, 48%, 47%, and 43%, respectively. Multivariate analysis showed that obesity was associated with higher NRM (HR 1.43, p=0.04), and lower relapse (HR 0.65, p=0.002). Pre-transplant plasma levels of ST2 and TNFR1 biomarkers were significantly higher in obese compared with normal weight patients (p=0.04 and p=0.05, respectively). The increase in NRM observed in obese patients was partially offset by lower incidence of relapse with no difference in overall survival.