The present study aimed at determining the relative distribution of cholesterol between the vesicular and micellar phases in gallbladder bile of gallstone patients (n = 23) and gallstone-free subjects (n = 7). Nine of the gallstone patients were treated with chenodeoxycholic acid and seven were treated with ursodeoxycholic acid, 15 mg/kg/day, for 3 wk before cholecystectomy. The vesicular and micellar fractions in bile were separated by sucrose density gradient ultracentrifugation, and a clear separation between the two phases was obtained. The vesicles were further identified by quasielastic light scattering spectroscopy and appeared to be of a uniform size with a mean hydrodynamic radius of 760 Å. The proportion of cholesterol in the vesicular fraction was significantly higher in the untreated gallstone group (40% ± 4%) compared with the gallstone-free (28% ± 3%), ursodeoxycholic acid (28% ± 3%) and chenodeoxycholic acid (18%± 4%) groups. Despite a low cholesterol saturation of bile in the latter three groups (88% ± 12%, 51% ± 9% and 65% ± 5%, respectively), a considerable part of the biliary cholesterol was carried in the vesicular fraction. The cholesterol/phospholipid ratio in the vesicular fraction averaged between 0.49 and 0.58 in the gallstone, gallstone-free and chenode-oxycholic acid groups, whereas the ursodeoxycholic acid group had a significantly lower ratio of 0.24. The cholesterol/phospholipid ratio was about 0.15 in all micellar fractions. The nucleation time of bile from the gallstone group was short (2 ± 1 days) compared with the gallstone-free, chenodeoxycholic acid and ursodeoxycholic acid groups (23 ± 3, 24 ± 6 and 14 ± 3 days, respectively). These observations give further support to the concept that the mechanism for gallstone dissolution is complex and different for chenodeoxycholic acid and ursodeoxycholic acid. (HEPATOLOGY 1991;13:104–110).