Fifteen patients in whom various parts of the small intestine had been resected because of Crohn's disease or mesenteric thrombosis and 10 healthy volunteers were studied. A 50-g oral glucose load (OGTT) and an intravenous glucose infusion giving the same plasma glucose profile as the OGTT were carried out to study (a) the relation between the plasma gastric inhibitory polypeptide (GIP) levels after oral glucose and the length and nature of the intestinal residues and (b) the importance of endogenous GIP as an incretin in man. The magnitude of the increase in plasma GIP after oral glucose load was positively correlated to the length of residual jejunum. The incretin effect was positively correlated to the length of residual intestine. Patients with preserved ileal residues had larger incretin effects than patients with less than 150 cm jejunal residues and no ileal residues, although the integrated increases in plasma GIP after oral glucose were equal. Compared with healthy volunteers, the patients with more than 150 cm residual jejunum had significantly higher increases in plasma GIP and normal incretin effects. The GIP release and the incretin effect in patients with preserved ileal residues were normal. The incretin effect of the patients with less than 150 cm jejunum was significantly subnormal in spite of a normal GIP release. These findings indicate that the upper intestine releases GIP after oral glucose and that other as yet unknown intestinal hormonal factors act as incretins in concert with GIP.