Supersaturation of bile with cholesterol predisposes to the development of cholesterol gallstones. To identify the factors determining cholesterol saturation of bile, we analyzed the lipid composition of stimulated duodenal bile in 60 healthy subjects of various ages (31 men and 29 women) who were not obese and were free of gallstones. A positive correlation between age and cholesterol saturation of bile was found (P less than 0.001). To analyze the relation between age and cholesterol saturation, we studied the rates of hepatic secretion of biliary lipids and the kinetics of cholic acid and chenodeoxycholic acid in 22 and 18 of the subjects, respectively. Age was positively correlated with the cholesterol secretion rate (r = 0.48) and negatively correlated with bile acid synthesis (r = -0.60) and the size of the cholic acid pool (r = -0.54). We conclude that cholesterol saturation of bile increases with age as a consequence of enhanced hepatic secretion of cholesterol and decreased bile acid synthesis. These findings may explain why age is a risk factor for the development of cholesterol gallstones.