Relative rates of cholesterol and bile acid synthesis were estimated in patients with cholesterol gallstones and biliary obstruction by determining the hepatic activities of 3-hydroxy-3-methylglutaryl-CoA reductase and cholesterol 7 alpha-hydroxylase, the respective rate-determining enzymes for cholesterol and bile acid synthesis. As compared with eight control studies, 3-hydroxy-3-methylglutaryl-CoA reductase activity was 27% higher in 12 gallstone subjects, but 75% lower in 5 subjects with biliary obstruction. Cholesterol 7 alpha-hydroxylase activity was reduced in the gallstone (47% lower) and biliary obstruction (78% lower) subjects. Liver cholesterol concentrations were 56% higher in the gallstone and 53% higher in the biliary obstruction subjects than the control group. These findings suggest that the pathogenesis of gallstones is related to both increased cholesterol synthesis and decreased bile acid formation, whereas cholesterol accumulates in biliary obstruction because of defective removal since cholesterol production is low.