The National Cooperative Gallstone Study, a double-masked, placebo-controlled, therapeutic trial of chenodiol (chenodeoxycholic acid), provided an opportunity to study the natural history of cholelithiasis in patients who choose nonsurgical management. The major component of the study comprised 916 patients, 305 of whom were randomly assigned to receive a placebo for 24 months. Among these 305 patients, the probability of having biliary tract pain during the 24 months of prospective evaluation was significantly increased if the patient had had a history of biliary tract pain in the 12 months before entry into the study (69% versus 31%). Thirty-eight percent of patients had stone growth (greater than 0.5 cm3), and 18% had a spontaneous decrease in stone volume. Despite the high incidence of biliary tract pain, nonelective cholecystectomy was required in only 4% of patients during the 24 months.