Reports on increased duodenal ulceration after bile diversion prompted the present investigation of gastric acid secretion and gastrin in 16 patients with a Roux-en-Y hepaticojejunostomy and 11 patients with a choledochoduodenostomy. Basal and pentagastrin stimulated acid output, gastrin in serum and gastrin in the antral mucosa were all significantly elevated in patients with a Roux-en-Y compared with those patients having a choledochoduodenostomy. In patients with a Roux-en-Y, bile bypassed the duodenum and the most proximal part of the jejunum; it is hypothesized that the bypass of bile may induce gastric acid hypersecretion secondary to an altered biosynthesis and release of gastrin.