Ileal disease or resection causes bile salt malabsorption and a reduction in the bile salt content of bile. Since cholesterol solubility requires adequate bile salt concentrations, depletion of the bile salt content of bile might, therefore, jeopardize cholesterol solubility and predispose to cholesterol gallstone formation. To study this, we examined biliary lipid composition in 10 patients with ileal dysfunction and in 25 healthy controls. Biliary lipid composition, as analysed in cholecystokinin-stimulated, bile-rich duodenal fluid, was shown to be representative of gallbladder bile and reproducible on repeat duodenal intubation. Nine of the 10 patients with ileal dysfunction had an abnormal, supersaturated bile in which the limits of cholesterol solubility were exceeded, and while nine of 25 control subjects also had an unstable bile, the mean bile composition in the ileal dysfunction group was significantly different from the control population. These studies provide a physicochemical explanation for the clinical observation that patients with ileal dysfunction have an increased incidence of gallstones.