A sample of 126 patients with cholelithiasis was treated with chenodeoxycholic acid (CDCA) (375 or 750 mg g.d.) for 2 years. Hepatotoxicity was assessed by sequential light (LM) and electron microscopic (EM) analysis of liver biopsies obtained before and after 9 and 24 months of therapy. Patients were without symptoms of biliary colic for 3 months prior to biopsy, less than 70 years old, and had normal tests of liver function. All light microscopy was read blindly by two morphologists, and electron microscopy by one. Only four biopsies (3%) showed severe (3+) abnormalities preventing admission to the study. On evaluation, moderate (2+) LM changes were present in less than 12% of biopsies, while milder changes (1+) were seen more frequently. No severe, dose-related abnormalities were seen in LM over the 2 years of study. However, mild changes (hepatocyte ballooning and lipofuscin, binucleate cells, glycogen nuclei, ductular proliferation, enlarged and fibrotic triads, and sinusoidal congestion) became more prevalent with time (p less than or equal to 0.01) regardless of CDCA dose, according to 1 of the 2 morphologists. Because patients with "proven" cholelithiasis for greater than 2 years (33% of the study population) did not have a greater incidence of morphologic abnormalities than those with the diagnosis for less than 2 years, it is unlikely that these changes were due to the natural history of gallstone disease. Furthermore, electron microscopic findings demonstrated worsening intrahepatic cholestasis and a lesion in two patients which was identical to that produced in animals administered lithocholic acid, the 7-alpha-dehydroxylated metabolite of CDCA. the possibility that clinically significant hepatotoxicity could develop with CDCA therapy greater than 24 months thus cannot be excluded by this study.