This retrospective analysis of 2503 adult (age ≥20 years) allogeneic hematopoietic cell transplantation (HCT) recipients assessed the effect of body mass index (BMI) on transplantation outcomes. The median patient age was 51.7years. Patients with both nonmalignant and malignant diagnoses were included. Patients received either a myeloablative (52%) or a reduced-intensity (48%) conditioning regimen. Donors were either related (42%) or unrelated (58%). Cord blood recipients were excluded. Granulocyte colony-stimulating factor-mobilized peripheral blood cells were the stem cell source in 86% of transplantations. Graft-versus-host disease prophylaxis included at least 2 immunosuppressive agents, 1 of which was a calcineurin inhibitor. Patient groups were categorized as underweight, normal weight, overweight, obese, or very obese based on BMI. Endpoints included day +100 mortality, overall mortality, nonrelapse mortality (NRM), and relapse. Changes in nutritional status, based on laboratory parameters, were also examined. Underweight patients had significantly lower early and overall survival and greater NRM. Very obese patients had increased NRM, which was associated with the intensity of conditioning regimen. With long-term follow-up, increasing NRM was associated with both underweight and obese patients compared with normal-weight individuals. Changes in serum protein and albumin levels did not correlate with BMI. Although enteral nutrition is now recommended for some undernourished patients, the efficacy of enteral or parenteral nutrition has not been well studied. For obese patients, there are no guidelines regarding weight loss before transplantation, and acute weight loss in the pretransplantation period may be detrimental.