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Background —Obesity is frequently cited as a risk factor for adverse outcomes of major surgery. The results of prior studies of the relationship between obesity and risk of adverse outcomes of coronary artery bypass grafting (CABG) have been contradictory because of insufficient power to assess relatively infrequent outcomes or data to adjust for confounding factors. Methods and Results —Data on patient age, sex, height, weight, medical history, current clinical status, and treatment factors were assessed prospectively among 11 101 consecutive patients undergoing CABG. Body mass index (BMI) was used as the measure of obesity and was categorized as nonobese (1st to 74th percentiles), obese (75th to 94th percentiles), or severely obese (95th to 100th percentiles). Adverse outcomes occurring in-hospital, including mortality, intraoperative/postoperative cerebrovascular accident (CVA), postoperative bleeding, and sternal wound infection, were defined prospectively. Associations between obesity and postoperative outcomes were assessed by use of logistic regression to adjust for potentially confounding variables. Although obesity was not associated with increased mortality (adjusted odds ratio [OR], 1.16; P =.261) or postoperative CVA (adjusted OR, 1.06; P =.765), risks of sternal wound infection were substantially increased in the obese (adjusted OR, 2.10; confidence interval [CI], 1.45 to 3.06; P <.001) and severely obese (adjusted OR, 2.74; CI, 1.49 to 5.02; P =.001). On the other hand, rates of postoperative bleeding were significantly lower in the obese (adjusted OR, 0.66; CI, 0.49 to 0.90; P =.009) and severely obese (adjusted OR, 0.40; CI, 0.20 to 0.81; P =.011). Conclusions —With the exception of sternal wound infection, the perception among clinicians that obesity predisposes to various postoperative complications with CABG is not supported by these data. Further work is needed to understand the apparent protective effect of obesity on risks of postoperative bleeding.