Spirometry and arterial blood gases were determined before operation and at approximately 6-month postoperative intervals in 11 patients weighing 137 +/- 22 kg (234% +/- 41% of actuarial ideal weight), who underwent a small bowel bypass which left 30 cm of proximal jejunum and 20 cm of distal ileum in end-to-end continuity. The average follow-up period was 20 months, during which time mean weight loss was 31% +/- 8%. Preoperative spirometry was normal, but the patients were significantly hypoxic (PaO2 = 75.3 +/- 8.5 mm Hg) as compared with an age-matched norm (P less than 0.001). PaCO2 and pH were normal. After operation changes in forced vital capacity (FVC) showed a significant linear correlation with the percentage of body weight lost: y = 1.03 x -29 (r = 0.75, P less than 0.001). Because of a significant (P less than 0.001) decrease in FVC, identified in seven patients, during the early period of rapid weight loss, the x intercept occurred at a weight loss of 27.5%. Additional weight reduction was attended by progressive improvement in FVC to values slightly higher, but not significantly different, than preoperative measurements. Forced expiratory volume at 1 second (FEV) varied in accord with FVC. Hypoxia persisted after operation and was unaffected by the amount of weight lost. The results suggest that loss of lean body mass, during the initial phase of rapid weight loss after intestinal bypass, may be detrimental to respiratory function, until a sufficient excess of fat also has been lost to achieve net benefit.