A high-fiber diet has been suggested as one reason for the low incidence of gallstones in some populations. Therefore the authors tested the effects of a high-fiber diet on biliary secretory kinetics and bile salt kinetics in morbidly obese volunteers. Bile salt pool sizes were reduced by 50%, and their half-lives were decreased 70% after the subjects had spent 6 weeks on the high-fiber diet. Bile acid enterohepatic circulation times also were shortened dramatically. However, the lithogenicity of bile did not decrease, and bile remained supersaturated with with cholesterol. Fasting bile samples tended to be even more lithogenic than before the subjects follwed the diet. In these obese subjects, a high-fiber diet failed to reduce the tendency to secrete cholesterol-saturated bile.