The feeding of one of the major biliary bile acids, chenodeoxycholic acid, at a dose of 10 to 15 mg/kg per day causes the circulating bile acid pool to become greatly enriched in this bile acid. When chenodeoxycholic acid composes more than 70% of the biliary bile acids, the amount of cholesterol secreted in bile falls, and bile becomes unsaturated in cholesterol. If cholesterol gallstones are present and are exposed to this unsaturated bile, they will dissolve in 4 to 24 months in the majority of patients. Extensive clinical experience indicates that such medical therapy is safe, despite unequivocal toxicity of chenodeoxycholic acid in several nonhuman primates. When therapy is stopped, bile resaturates, and stones may recur. Since cholecystecomy is a rapid, safe, effective, and usually permanent treatment for all gallstones, the value of medical therapy remains uncertain at present, except for patients in whom surgery is inadvisable. Nonetheless, the demonstration that chenodeoxycholic acid ingestion will desaturate bile and induce gallstone dissolution would appear to be an important pharmacological advance.