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Pulmonary function tests (PFTs) are often abnormal in the morbidly obese and improve after bariatric surgery. Our aim was to determine the utility of obtaining preoperative PFTs in assessing postoperative risk.146 consecutive patients undergoing open bariatric surgery were analyzed. Patients were divided into those who had postoperative complications (Group A, n=27) and those who did not (Group B, n=119). PFTs and BMI were compared between Groups A and B. PFT parameters are reported as the median percentage of age-matched controls.Patients in Group A compared to Group B were heavier (BMI 58 vs 51 kg/m2,P=.001) and older (46 vs 40 years,P=.02) than those in group B. They had reduced forced vital capacity (80% vs 97%,P<.001) and forced expiratory volume in 1 second (84% vs 99%,P=.002). They also had reduced vital capacity (VC, 85% vs 102%,P<.001) and total lung capacity (89% vs 99%,P=.01). They had decreased maximal voluntary ventilation (93% vs 106%,P=.003). They had lower arterial pO2 (76 mmHg vs 85 mmHg,P=.001) and higher arterial-alveolar gradient (23 vs 17,P=.007). The strongest predictors of postoperative complications on multivariate analysis were reduced VC (RR 2.29 for each 10% decrease in VC,P=.0007) and age (RR 6.4 for age >40 years,P=.01).PFTs help to predict complications after bariatric surgery. The greatest reduction in VC may occur in patients with central obesity, reflecting increased intrabdominal pressure and diminished chest wall compliance.