Nutritional status after 238 gastric operations designed to reduce caloric intake and body weight to within 30% of ideal was assessed by measuring body composition using the multiple isotope dilution technique. Body cell mass (BCM) and body fat were quantitated before and at 24 months after operation. Malnutrition was defined as a total exchangeable sodium (Nae) to total exchangeable potassium (Ke) ratio greater than 1.22. Data were collected on 96 patients. All had lost a mean of 26% of preoperative weight by 24 months. Significant malnutrition occurred in 47 patients whose Nae/Ke ratio ranged from 1.23 to 2.17 (1.45 +/- 0.03). There was a 34% reduction in body fat. The malnourished patients lost 10% more BCM by 24 months than did the normally nourished group. Malnutrition resolved as the stoma enlarged in 19 patients, and dietary counselling helped eight patients. Eighteen patients required reoperation to establish a larger orifice, and endoscopic dilatation was successful in two patients. Administration of a liquid diet via the gastrostomy was required for prolonged periods in some malnourished patients. Seventeen patients who had lost weight rapidly over a short time had low vitamin B12, thiamine, and serum and RBC folate levels. One patient had a markedly decreased serum thiamine level with neuropathy. Symptoms of weakness, easy fatigability, and lassitude were found in the malnourished patients. Low thiamine and serum folate levels were also seen in patients ingesting a liquid diet of 750 kcal with a standard multivitamin supplement. Malnutrition was not seen in these patients. In the 49 patients who remained well nourished, BCM decreased by 19%, but the Nae/Ke remained normal. Weight loss was well tolerated, and no patients required reoperation or supplemental liquid diet to increase caloric or protein intake. The degree of malnutrition in patients after gastric operations is as great as following intestinal bypass but is not associated with liver failure. Malnutrition with vitamin deficiency is a great potential hazard in patients who undergo intake-limiting operations, especially if the goal of the operation is to restore near-normal weight. Current operations are successfully designed to maintain a small orifice size, so that the risks of malnutrition are likely to increase in the future.