Background More than one third of adults in the United States are obese. Coronary artery bypass graft surgery (CABG) has become necessary for many obese persons. We evaluated the effect of this procedure on in-hospital mortality and morbidity of patients based on their body mass index (BMI). Methods Data in a cardiac surgery database were examined retrospectively. Data selected from the database included CABG surgery from January 2003 to December 2007. The resulting cohort included a total of 10,590 patients. The BMI was grouped into four categories: underweight (BMI ≤ 19), normal weight (BMI 20 to 29), obese (BMI 30 to 39), and morbidly obese (BMI ≥ 40). Regression analysis was conducted to determine whether BMI was an independent predictor of morbidity and mortality after CABG. Results Our results indicate that patients with an obese BMI are not at greater risk for morbidity or mortality after CABG. Logistic regression analysis found that CABG patients in the underweight body mass index group had the greatest risk of mortality, prolonged ventilation, reoperation for bleeding, and renal failure. Linear regression indicated length of hospital stay and intensive care unit stay after surgery were the longest for patients with an underweight BMI. Conclusions Despite the comorbidities that are often present with obesity, an obese BMI was not found to be an independent predictor of morbidity or mortality after CABG. On the contrary, the underweight patients are at greater risk for mortality and complications after CABG surgery.