A simple method for the assay of biliary lipids was used for routine determination of biliary cholesterol saturation index (CSI) in 17 healthy subjects, 40 untreated patients with radiolucent gallstones and 21 gallstone patients treated with ursodiol. The method is based on collection of bile with the Entero-Test, a device for easy sampling of gastrointestinal contents. The procedure was easy to perform and well accepted by the patients. Both CSI and the cholesterol content of bile were higher in untreated gallstone patients than in controls, and were significantly lower in treated than in untreated patients. Normal CSI was found in 35% of untreated gallstone patients, while 24% of healthy subjects had supersaturated bile. Supersaturated bile was still present in 14% of ursodeoxycholic acid treated patients, suggesting inefficacy of therapy. These results demonstrate the possibility of performing easy, routine determination of biliary lipid composition, which can yield useful information for the management of gallstone patients.