Changes in bile saturation and biliary bile acid composition in patients with gallstones who received chenodeoxycholic ("chenic") acid, cholic acid, or placebo were measured. Chenodeoxycholic induced bile desaturation; this effect was attributable solely to a decrease in the proportion of cholesterol. By gas chromatography, chenodeoxycholic acid increased substantially in the biliary bile acids of patients receiving it, and by mass spectrometry, no unusual bile acids were detected in appreciable amounts. Changes in bile saturation and biliary bile acid composition were then related to chenodeoxycholic acid dosage, and all of these variables were, in turn, related to gallstone response. In general, patients whose gallstones dissolved ingested a higher dose of chenodeoxycholic acid or had bile that contained a higher proportion of this acid and it was more unsaturated, but there were many exceptions, casting doubt on the value of a single analysis of fasting-state bile for predicting gallstone dissolutions. The major factor influencing response, provided dosage is adequate, appears to be gallstone type. Nonetheless, the proportion of chenodeoxycholic acid in biliary bile acids can probably be used to infer patient compliance.