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A scoring system for clinical assessment of mortality risk has been previously proposed for bariatric surgery (Demaria EJ, Portenier D, Wolfe L, Surg Obes Relat Dis. 2007;3:34–40.). The Obesity Surgery Mortality Risk Score (OS-MRS) was developed from a single institution experience of 2075 patients. The current study provides multicenter validation of the value of the OS-MRS. The OS-MRS assigns 1 point to each of 5 preoperative variables, including body mass index ≥50 kg/m2, male gender, hypertension, known risk factors for pulmonary embolism (previous thromboembolism, preoperative vena cava filter, hypoventilation, pulmonary hypertension), and age ≥45 years. Patients with total score of 0 to 1 are classified as ‘A’ (lowest) risk group, score 2 to 3 as ‘B’ (intermediate) risk group, and score 4 to 5 as ‘C’ (high) risk group.Prospectively-collected data from 4431 consecutive patients undergoing a primary gastric bypass at 4 bariatric programs recruited to validate the proposed system were analyzed to assess OS-MRS as a means of stratifying surgical mortality risk.There were 33 total deaths for an overall mortality for the validation cohort of 0.7% consistent with published standards. Mortality for 2164 class A patients was 0.2%, for 2142 class B patients was 1.1%, and for 125 class C patients was 2.4%. Mortality was significantly different between each of the class A, B, and C groupings (P < 0.05, χ2). Mortality was 5-fold greater in the class B group than in class A. Only 6 patients with all 5 risk factors were identified. Class C patients (n = 125, 3% of total cohort) were characterized by a 12-fold greater mortality than the lowest risk group (A) and a disproportionate 9% of all mortalities.The OS-MRS was found to stratify mortality risk in 4431 patients from 4 validation centers that were nonparticipants in the original defining cohort study. The score represents the first validated scoring system for risk stratification in bariatric surgery and is anticipated to aid informed consent discussions, guide surgical decision-making, and allow standardization of outcome comparisons between treatment centers.