['The prevalence of the metabolic syndrome and nonalcoholic fatty liver disease (NAFLD) in humans increases with age. It is unknown whether this association is secondary to the increased incidence of risk factors for NAFLD that occurs with aging, reflects the culmination of years of exposure to lifestyle factors such as a high-fat diet (HFD), or results from physiological changes that characterize aging. To examine this question, the development of NAFLD in response to a fixed period of HFD feeding was examined in mice of different ages. Mice aged 2, 8, and 18 months were fed 16 weeks of a low-fat diet or HFD. Increased body mass and insulin insensitivity occurred in response to HFD feeding irrespective of the age of the mice. The amount of HFD-induced hepatic steatosis as determined biochemically and histologically was also equivalent among the three ages. Liver injury occurred exclusively in the two older ages as reflected by increased serum alanine aminotransferase levels, positive terminal deoxynucleotide transferase-mediated deoxyuridine triphosphate nick end-labeling, and caspase activation. Older mice also had an elevated innate immune response with a more pronounced polarization of liver and adipose tissue macrophages into an M1 phenotype. Studies of cultured hepatocytes from young and old mice revealed that aged cells were selectively sensitized to the Fas death pathway.'] Does aging promote the development of diet-induced murine steatohepatitis but not steatosis?