Bile specimens were obtained from 17 patients with gallstones and 21 patients with duodenal ulcer. The specimens were obtained from the former by needle aspiration of the gallbladder and common bile duct at operation and from the latter by duodenal intubation. The concentrations of bile salt, phospholipid, and cholesterol were measured. Gallbladder bile from gallstone patients contained significantly more cholesterol than did ;duodenal' bile from duodenal ulcer patients. Hepatic bile from gallstone patients contained significantly more cholesterol than did gallbladder bile from the same patients. When the data were plotted on triangular coordinates the relative composition lay within the zone of cholesterol solubility in all 21 ulcer patients. The relative composition of hepatic bile lay outside the zone of cholesterol solubility in five gallstone patients, at the limits of cholesterol solubility in a further three, and within the micellar zone in the remaining nine patients. This suggests that supersaturation of hepatic bile with cholesterol is not the sine qua non for the production of cholesterol gallstones.