The relation between malabsorption of bile acids, the bile lipid composition, and biliary stones was examined in 8 patients subjected to ileal resection (particularly for Crohn's disease), 6 with ileal bypass for morbid obesity, and 10 healthy controls. The 1-14C-cholylglycine breath test was employed to detect of the absorption and deconjugation of bile acids. Bile lipid composition was expressed according with Metzger's saturation index. Healthy subjects gave normal findings in all respects, whereas ileal resection was accompanied by malabsorption, increased deconjugation, and faecal loss of 14C. These changes, particularly malabsorption, were more evident after ileal bypass. Preoperative saturation values rose to more than 1 in all cases, especially after resection. Liver disease (steatosis and cirrhosis) 6 months after bypass, together with cholesterol lithiasis in 2/6 patients.