To determine how choline supplementation affects the liver and whether it can protect against ethanol-induced liver injury, baboons were fed either normal or choline-supplemented diets, each with or without ethanol. Eighteen baboons were pair-fed for 3 to 4 years liquid diets with 50% of total energy as ethanol or isocaloric carbohydrate; ten animals were given our regular diets, whereas in eight the choline content was increased 5-fold. Six additional animals were fed individually with the control diets (with or without additional choline). With both ethanol-containing diets, ethanol intake was comparable and resulted in hepatic steatosis and striking mitochon-drial lesions, with increases in serum biliburin and SGOT, SGPT and glutamate dehydrogenase activities. In addition, of the five animals fed alcohol with the regular diet, one progressed to incomplete cirrhosis and two others developed perivenular and associated perisinusoidal fibrosis. Similarly, in the four baboons fed alcohol with choline supplementation, incomplete cirrhosis developed in one and perivenular fibrosis in two. Collagen deposition was demonstrated by immunoperoxidase with a specific antibody against procollagen Type III. These animals also displayed proliferation of myofibroblasts in the perivenular area and transformation of fat-storing cells to transitional cells in the perisinusoidal space, with associated enhanced collagen fiber deposition. Thus, in baboons, choline supplementation failed to prevent alcohol-induced steatosis and fibrosis. All parameters remained normal in the eight baboons fed the regular control diet. However, in the choline-supplemented controls, serum bilirubin, SGOT and glutamate dehydrogenase activities increased moderately and serum albumin decreased. Occasional fat droplets appeared in hepatocytes with mitochondrial changes (enlargement and alterations of the cristae) and an abundance of “myelin” figures in the cytoplasm, indicating that choline supplementation exerts moderate hepatotoxicity.