Limited data have suggested that an “obesity paradox” exists for mortality and cardiovascular outcomes in patients undergoing coronary artery bypass grafting. Much less is known about the role of the preoperative body mass index (BMI) in patients undergoing valve surgery. We evaluated 2,640 consecutive patients who underwent valve surgery between April 2004 and March 2011. The patients were classified by the World Health Organization standards as “underweight” (BMI 11.5 to 18.4 kg/m², n = 61), “normal weight” (BMI 18.5 to 24.9 kg/m², n = 865), “overweight” (BMI 25 to 29.9 kg/m², n = 1,020), and “obese” (BMI 30 to 60.5 kg/m², n = 694). Mortality was ascertained using the Social Security Death Index. Hazard ratios (HRs), adjusted for known preoperative risk factors, were obtained using Cox regression models. The mean follow-up was 31.9 ± 20.5 months. The long-term mortality rate was 1.21, 0.52, 0.32, and 0.44 per 10 years of person-time for underweight, normal, overweight, and obese patients, respectively. Compared to the normal BMI category, overweight patients (adjusted HR 0.60, 95% confidence interval 0.46 to 0.79, p <0.001) and obese patients (adjusted HR 0.67, 95% confidence interval 0.50 to 0.91, p = 0.009) were at a lower hazard of long-term all-cause mortality. Underweight patients remained at a greater adjusted risk of long-term mortality than normal weight patients (adjusted HR 1.69, 95% confidence interval 1.01 to 2.85, p = 0.048). Similar patterns of mortality outcomes were noted in the subset of patients undergoing isolated valve surgery. In conclusion, overweight and obese patients had greater survival after valve surgery than patients with a normal BMI. Very lean patients undergoing valve surgery are at a greater hazard for mortality and might require more rigorous preoperative candidate screening and closer postoperative monitoring.