Abstract INTRODUCTION: Deviation from normal body size, particularly extreme obesity is associated with increased postoperative morbidity and mortality. Obesity is often perceived to be a risk factor for adverse outcomes following coronary artery surgery bypass graft. The aim of this study was to evaluate the effect of body mass index on the early outcomes in patients undergoing coronary artery bypass graft (CABG). methods: In a retrospective study, 772 patients undergoing CABG between 2005 and 2006 were evaluated in shiraz university affiliated medical centers. The patients’ body mass index (BMI) was measured and classified as underweight, normal-weight, obese and severely obese. The clinical data were evaluated with respect to early postoperative outcomes and mortality. The main early outcomes were postoperative myocardial infarction, sternal wound infection, respiratory and renal problems, atrial arrhythmia, bleeding, longer duration of mechanical ventilation, prolonged hospital stay, and increased operative mortality. The data were gathered using a demographic information form and a checklist to determine the number of main early outcomes. The forms were completed with data from the patients and their records. The effect of BMI on the early outcomes in patients undergoing coronary artery bypass graft (CABG) was assessed using odds ratio and the logistic regression model. results: The results showed that of 772 patients, 13.6%, 2.6%, 75.4%, and 8.4% were obese, severely obese, normal-weight, and underweight, respectively.. Obesity and severe obesity increased the risk of sternal wound infection (odds ratio=9.761, P<0.001 and odds ratio=34.441, P<0.001, respectively). Obesity increased the risk of atrial arrhythmia (odds ratio=5.173, P<0.001). Obesity and severe obesity were significantly associated with respiratory problems and postoperative stay longer than 14 days. Severe obesity increased the risk of operative mortality (odds ratio=15.390, P<0.001). There was no difference between obese and severely obese patients in respect of the incidence of myocardial infarction, renal failure, and bleeding following operation. CONCLUSIONS: Obesity and severe obesity are associated with increased early complications and operative mortality after CABG, and the degree of obesity plays a key role in adverse outcomes of this procedure.. Hence, weight loss can contribute to reduction of postoperative CABG complications and mortality. Keywords: Body mass index, coronary artery bypass graft, obesity, morbidity, mortality.