Background: Obesity is associated with poor surgical outcomes and disparity in access-to-care. There is a lack of quality data on the effect of body mass index (BMI) on perioperative outcomes. Accordingly, we sought to determine the procedure specific, independent-effect of BMI on 30-day perioperative outcomes in patients undergoing major surgery. Methods: Participants included individuals undergoing one of 16 major surgery (cardiovascular, orthopedic, oncologic; n = 141,802) recorded in the ACS-NSQIP (2005-2011). Outcomes evaluated included complications, blood transfusion, length-of-stay (LOS), re-intervention, readmission, and perioperative mortality. Multivariable-regression models assessed the independent-effect of BMI on outcomes. Results: Nearly, 74 % of patients had a BMI disturbance; the majority being overweight (35.3 %) or obese (29.8 %). Morbidly obese patients constituted a small but significant proportion of the patients (5.7 %; n = 8067). In adjusted-analyses, morbidly obese patients had significantly increased odds of wound complications in 15 of the examined procedures, of renal complications after 6-procedures, of thromboembolism after 5-procedures, of pulmonary, septic and UTI complications after 2-procedures, and of cardiovascular complications after CABG. Conversely, obese/overweight patients, except for increased odds of wound complications after select procedures, had significantly decreased odds of perioperative mortality, prolonged-LOS and blood transfusion relative to normal BMI patients after 4, 8, and 9 of the examined procedures. Conclusions: The prevalence of BMI derangements in surgical patients is high. The effect of BMI on outcomes is procedure specific. Patients with BMI between 18.5 and 40-kg/m2 at time of surgery fare equally well with regard to complications and mortality. However, morbidly obese patients are at-risk for postsurgical complications and targeted preoperative-optimization may improve outcomes and attenuate disparity in access-to-care.