Because of the increased incidence of gallstones in cystic fibrosis we compared biliary lipid composition in 26 patients with cystic fibrosis, seven children with cholelithiasis but no cystic-fibrosis and 13 controls. Eighteen of the cystic fibrosis group had cholecystograms, and only one had gallstones. In 14 patients with cystic fibrosis who had stopped taking pancreatic enzymes for one week molar percentage of lipid composition accounted for by cholesterol (mean +/- S.E., 16.3 +/- 2.9) and saturation index (2.0 +/- 0.3) were comparable to values of the cholelithiasis group and higher (P less than 0.01) than those of controls. In 12 patients with cystic fibrosis taking pancreatic enzymes, molar percentage of cholesterol (8.6 +/- 1.7) and saturation index (1.0 +/- 0.1) did not differ from those of controls; in cystic fibrosis there was a preponderance of cholic over chenodeoxycholic acid both off (1.7 +/- 0.2) and on (1.9 +/- 0.3) therapy as compared to the cholelithiasis (0.7 +/- 0.1) and control (0.8 +/- 0.0) groups. The glycine/taurine ratio of conjugated bile acids were lower in enzyme-treated patients with cystic fibrosis (3.7 +/- 0.6) than in patients off treatment (6.4 +/- 1.0), but was higher (P less than 0.01) than in controls (1.8 +/- 0.2). Bile is lithogenic in untreated cystic fibrosis and responds to pancreatic enzymes.