Stress hyperglycemia and acute graft-versus-host disease (GVHD), the major early complication of hematopoietic stem cell transplantation (HSCT), are both associated with excessive release of inflammatory cytokines. We investigated whether new-onset hyperglycemia immediately after HSCT predicts acute GVHD. We studied nondiabetic adult recipients of human leukocyte antigen-matched HSCT (peripheral blood stem cells) for acute leukemia. Using mean morning serum glucose on Days 1-10, we classified hyperglycemia as: mild (6.11-8.33 mmol/L), moderate (8.34-9.98), and severe (minimum of 9.99). Subjects who were GVHD-free on Day 10 were followed during Days 11-100 for grades II-IV acute GVHD or competing event. Evaluation utilized cumulative incidence-based proportional hazards regression. Subjects (n = 328) were age 18-74, median of 49 years. Per body mass index (BMI)--25.0 % were obese (BMI, 30-48), 33.8 % overweight (25 to <30), 30.8 % normal weight (21 to <25), and 10.4 % lean (18 to <21). Mild, moderate, or severe hyperglycemia occurred during Days 1-10 in 50.0, 21.3, and 16.8 % of subjects, respectively. Cumulative incidence on Day 100 was 44.8 (±2.8) % acute GVHD and 7.9 (±1.5) % competing event. Among normal-to-overweight subjects (n = 212), severe hyperglycemia developed in 14.2 % (n = 30) and more than doubled the risk of acute GVHD (hazards ratio, 2.71; 95 % CI, 1.58-4.65--adjusted for donor/recipient characteristics, prophylactic regimen, and mucositis). In contrast, among obese subjects (n = 82), severe hyperglycemia developed in 30.5 % (n = 25) but did not significantly affect risk of GVHD. (No lean subjects (n = 34) developed severe hyperglycemia.) Hyperglycemia that was less than severe had an effect indistinguishable from normoglycemia. In nondiabetic patients, severe hyperglycemia immediately after allogeneic HSCT indicates increased likelihood of acute GVHD. This association is absent in obese patients, who may be primed by obesity-induced inflammation to develop severe hyperglycemia even without experiencing the cytokine storm that is essential to GVHD pathogenesis.