To elucidate mechanisms involved in weight development after gastrectomy we have prospectively determined changes in body composition during the first year after similar operations. A total of 75 patients were enrolled who had a "curative operation" for gastric carcinoma; 42 were randomized to have a total gastrectomy, 23 total gastrectomy with a gastric substitute, and 10 subtotal gastrectomy. All reconstructions were done with a Roux-en-Y loop of the jejunum. Body composition was assessed preoperatively and at 6 and 12 months after gastrectomy by determining total body potassium and total body water. From these estimates, body cell mass, extracellular water, fat-free extracellular solids, and body fat were calculated with knowledge of the actual body weight and length. Triceps skinfold, arm muscle circumference, and grip strength were also measured. Weight loss (10% of preoperative weight) occurred early after the operations, after which body weight stabilized. Body cell mass remained essentially unchanged over the entire study period in contrast to body fat, which decreased by 40% during the first 6 months after gastrectomy. In accordance with the selective loss of body fat, we recorded a significant decrease in triceps skinfold figures and only a minor decrease of arm muscle circumference without obvious deterioration in hand grip strength. Weight loss after gastrectomy seems to be characterized by selective loss of body fat in contrast to other known clinical situations associated with impaired nutritional intake. Our observations form a basis for future clinical research aimed at preventing weight loss after these operations.