The effects of Acipimox (5-methylpyracine-2-carboxylic acid-4-oxide, 3 X 250 mg/day) on plasma lipids, lipoproteins, and biliary lipids were studied in 14 healthy male volunteers using a double blind cross-over design. There was a significant (P less than 0.05) rise of HDL-cholesterol by 9.8%, while effects on other lipids and lipoproteins were small and insignificant (total cholesterol minus 6.5%, LDL-cholesterol minus 11.8%, free cholesterol minus 4.2%, total triglycerides plus 13.5% and phospholipids plus 3.9%). There was a significant rise of the HDL-cholesterol/LDL-cholesterol ratio by 23.6% (P less than 0.02) and of the HDL-cholesterol/total cholesterol ratio by 16% (P less than 0.05). Apolipoproteins AI, AII, and B were not significantly affected. The ratio of HDL-cholesterol/Apo AI increased significantly (P less than 0.05), and the ratio of HDL-cholesterol/Apo AII rose from 1.22 to 1.32 (P less than 0.05), while the ratio LDL-cholesterol/Apo B fell from 1.96 to 1.73. The composition of HDL and LDL, therefore, must have been altered by Acipimox. The relative cholesterol concentration in bile was significantly (P less than 0.05) increased by treatment with Acipimox, while bile acids and phospholipids were not significantly affected. The lithogenic index rose significantly by 15.1% (P less than 0.02) as calculated according to Admirand and Small, while calculations according to Hegard and Dam yielded a slight insignificant rise (P less than 0.1). The findings suggest that treatment with Acipimox might be associated with an increased risk of cholelithiasis. However, only long-term epidemiologic studies can ultimately demonstrate whether or not Acipimox increases the risk of gallstone formation.