Pulmonary function tests (PFTs) and arterial oxygenation were studied 24 hours preoperatively and again at varying times (5 to 20 months) during the postoperative weight loss of 37 morbidly obese patients. Among PFTs, expiratory reserve volume (ERV) demonstrated the most significant (P less than 0.001) improvement with weight loss. No clinically relevant differences were found after weight loss in FEV1, FVC, or MMEF. In 11 patients whose arterial blood gases were sampled before and after weight loss, a significant correlation was demonstrated between change in ERV and change in Pao2 (r = 0.59, P less than 0.05) and P(A-a)o2 (r = 0.76, P less than 0.01). When weight loss is expressed as either change in body mass index (delta BMI) or as a change in percent of ideal weight (delta wt), there existed a weight loss threshold for improvement in arterial oxygenation. A delta BMI of greater than 20 or a delta wt more than 100 per cent resulted in a significant improvement in Pao2 (P less than 0.01) or P(A-a)o2 (P less than 0.05). These data suggest that morbidly obese patients do not meaningfully improve FEV1, FVC, or MMEF with massive weight loss. However, there is a significant improvement in ERV that directly correlates with improvement in both Pao2, and P(A-a)o2.