Nonalcoholic fatty liver disease (NAFLD) is highly associated with the metabolic syndrome, and occurs as a more serious form of the disease, nonalcoholic steatohepatitis (NASH). NASH is diagnosed pathologically by histological evaluation of fibrosis, inflammation, and other features, such as hepatocyte ballooning. IL-19 is a member of the IL-10 family and is an anti-inflammatory cytokine produced mainly by macrophages. The last 10 years from the finding of IL-19, investigations underline the role of IL-19 in the immunological diseases such as inflammatory bowel disease, pancreatitis, and contact hypersensitivity. However, the involvement of IL-19 in liver inflammation and liver fibrosis is not well understood. We investigated the immunological role of IL-19 in NAFLD/NASH model mice fed by a choline-deficient and high-fat diet with methionine and cholesterol for 2 months. IL-19 knockout mice (KO) showed the major feature of NASH such as pericellular fibrosis, although wild-type mice showed steatohepatitis. In this symposium, we will report the recent advances in the role of IL-19 as inflammatory mediators in NASH. IL-19KO may be a valuable tool to study the NAFLD/NASH.