Metabolic parameters were studied in 6 patients before and for 18 months after jejunoileal bypass for obesity. The postoperative changes in plasma concentration of amino acids were characterized by a decrease in essential amino acids, while the non-essential amino acids were unchanged or elevated, indicating a state of protein-caloric malnutrition. The pattern of fatty acids in serum the first year after operation: an unchanged sum of unsaturated and saturated fatty acids accompanied by a significant fall in linoleic acid and a corresponding increase in oleic acid, showed that even during this period the patients were in a state of essential fatty acid deficiency. The oral glucose tolerance test and the levels of serum immunoreactive insulin activity indicated a general improvement in glucose tolerance and a normalization of preoperative hyperinsulinism. A 40-50% reduction was found both in serum triglycerides and cholesterol levels. Serum iron was reduced by approximately 30%, whereas TIBC levels remained unchanged. The Schilling test showed a significant fall in vitamin B12 absorption postoperatively with subnormal values at 6 and 12 months. Vitamin B12 in serum dropped, but remained within the normal range, suggesting that the preservation of 25 cm of distal ileum is sufficient to secure adequate absorption of vitamin B12. Folic acid levels in serum fell after the bypass, demanding supplementation in 5 of the 6 patients at 6 months postoperatively.