Biliary lipid composition was determined in fasting gallbladder bile of 12 young and 12 elderly asymptomatic Chilean women with normal weights and recent, normal cholecystograms. The proportion of biliary cholesterol and the lithogenic index were significantly higher in elderly females. Moreover, supersaturated bile was present in 8.3% of the young and in 41.7% of the older women studied. Obesity enhances the saturation of bile in older females: lithogenic bile was found in 100% of 4 elderly, obese women with radiologically normal gallbladders. The bile acid pool and cholic acid metabolism were studied in 5 young and 4 older normal females. Both groups showed similar values of bile acid pool, cholic acid synthesis, and turnover. These results indicate that aging per se modifies the proportions of biliary lipids in Chilean women, and provide a partial explanation for the frequency of gallstones observed among them. The increment in the lithogenic index of gallbladder bile with age takes place without significant changes in bile acid metabolism, suggesting that the canalicular secretion of cholesterol increases with aging.