Lipid-lowering drugs, notably clofibrate, may induce a supersaturation of bile with cholesterol, thus favouring the development of cholelithiasis. In order to see whether or not fenofibrate, a clofibrate analogue, has any influence on biliary cholesterol saturation, we determined the lipid composition of gallbladder bile and the bile acid pool size in 15 patients with type IV hyperlipoproteinemia before and after 6-8 weeks of treatment with a daily dose of 300 mg of the drug. At the end of treatment plasma triglycerides were markedly decreased, whereas no detectable influence on liver cell integrity or bile excretory function could be demonstrated in any patient by comparing the pre- and post-treatment serum levels of liver enzymes, bilirubin and bile acids. The mean bile cholesterol saturation index did not significantly change and cholic acid was the only bile acid to increase significantly. In the 3 patients with an initial saturation index of less than 1, bile became supersaturated with cholesterol. However, in no case were the limits of the metastable phase for cholesterol solubility in bile exceeded. Though only long-term prospective studies may give a definitive answer about the lithogenic potential of fenofibrate, our data on gallbladder bile composition in patients with type IV hyperlipoproteinemia indicate that it is not very likely that fenofibrate administration will increase the risk of gallstone formation in severely hyperlipidemic patients.