OBJECTIVES: This study sought to quantify the effect of body mass index (BMI) on early clinical outcomes following coronary artery bypass grafting (CABG). BACKGROUND: Obesity is considered a risk factor for postoperative morbidity and mortality after cardiac surgery, although existing evidence is contradictory. METHODS: A concurrent cohort study of consecutive patients undergoing CABG from April 1996 to September 2001 was carried out. Main outcomes were early death; perioperative myocardial infarction; infective, respiratory, renal, and neurological complications; transfusion; duration of ventilation, intensive care unit, and hospital stay. Multivariable analyses compared the risk of outcomes between five different BMI groups after adjusting for case-mix. RESULTS: Out of 4,372 patients, 3.0% were underweight (BMI <20 kg/m(2)), 26.7% had a normal weight (BMI >or=20 and <25 kg/m(2)), 49.7% were overweight (BMI >or=25 and <30 kg/m(2)), 17.1% obese (BMI >or=30 and <35 kg/m(2)) and 3.6% severely obese (BMI >or=35 kg/m(2)). Compared with the normal weight group, the overweight and obese groups included more women, diabetics, and hypertensives, but fewer patients with severe ischemic heart disease and poor ventricular function. Underweight patients were more likely than normal weight patients to die in hospital (odds ratio [OR] = 4.0, 95% CI 1.4 to 11.1), have a renal complication (OR = 1.9, 95% confidence interval [CI] 1.0 to 3.7), or stay in hospital longer (>7 days) (OR = 1.7, 95% CI 1.1 to 2.5). Overweight, obese, and severely obese patients were not at higher risk of adverse outcomes than normal weight patients, and were less likely than normal weight patients to require transfusion (ORs from 0.42 to 0.86). CONCLUSIONS: Underweight patients undergoing CABG have a higher risk of death or complications than normal weight patients. Obesity does not affect the risk of perioperative death and other adverse outcomes compared to normal weight, yet obese patients appear less likely to be selected for surgery than normal weight patients.