Patients who undergo gastric partitioning for morbid obesity lose weight rapidly in the first year after surgery. This study measured changes in body composition and dietary intake at 3 months, 6 months, and 1 year after gastric partitioning. Body composition was determined by total-body gamma-ray spectrometry. Records of dietary intake were computer analyzed for nutritional content with the Ohio State Nutrient Analysis Data Base. All patients lost a significant amount of weight at 3, 6, and 12 months after surgery. Loss of lean tissue comprised 25% of the total weight loss at 3 months but only 20% of the total at 1 year. Before surgery the mean energy intake was 2592 kcal and the mean protein intake was 92 gm. After 3 months, energy intake was 745 kcal and protein 32 gm. One year after surgery energy intake was 1089 kcal and protein 41 gm per day. We conclude that patients lose a significant amount of lean body tissue in the first 3 months after gastric partitioning but the loss does not continue, and 1 year after surgery some lean tissue is actually regained. This initial loss is due to a combination of a very restrictive dietary regimen and the body's metabolic response to surgery. After the first 3 months adaptation occurs and lean tissue is spared.