The influence of body mass on outcome after cardiac surgery remains controversial. The aim of this study was to analyze the impact of body mass index (BMI) on early and late outcomes in a large series of patients who underwent cardiac surgery. We retrospectively analyzed 5,950 consecutive patients who underwent cardiac surgery between January 1998 and September 2006. Patients were divided into 4 groups defined by BMI (weight divided by square of height [kilograms divided by meters squared]): underweight (<or=20 kg/m2): 6%, n=328; normal (>20 to <or=25 kg/m2): 34%, n=2,041; overweight (>25 to <or=30 kg/m2): 38%, n=2,289; obese (>30 kg/m2): 22%, n=1,292. Analysis was further refined by performing subgroup analysis according to the surgical procedure (valve surgery, coronary artery bypass grafting, and combined valve/coronary artery bypass grafting). Main outcome measure was the association between BMI and hospital mortality, postoperative morbidities, and late survival. Hospital mortality was 3.4% (n=203). There was no association between BMI and hospital mortality in the entire patient population. Multivariate analysis revealed obesity as an independent predictor of hospital mortality in patients who underwent valve surgery (odds ratio [OR] 2.3, 95% confidence interval [CI] 1.2 to 4.5, p=0.018). Obesity was associated with an increased risk for sternal infection (OR 1.8, 95% CI 1.1 to 2.9, p=0.013), whereas underweight correlated with postoperative bleeding (OR 2.0, 95% CI 1.1 to 3.6, p=0.017). Underweight was an independent predictor for decreased long-term survival (OR 1.8, 95% CI 1.3 to 2.5, p<0.001). In conclusion, cardiac surgery can be performed safely in both underweight and obese patients but carries a higher postoperative rate of major complications.