Background: Autoimmune hepatitis (AIH) is a rare progressive liver disease, which manifests as acute hepatitis in 40％-50％ of pediatric cases. This refers predominantly to spontaneous exacerbations of previ- ously unrecognized subclinical AIH with laboratory and histological signs of chronic hepatitis, or to acute exacerbations of known chronic disease. Only a few of these patients fulfill criteria for acute liver failure (ALF).Methods: Forty children diagnosed with AIH in our center between 2000 and 2018 were included in this study. All of them fulfilled revised diagnostic criteria of the Intational Autoimmune Hepatitis Group (IAIHG) for probable or confirmed AIH, and other etiologies of liver diseases were excluded. Patients were divided into two groups: acute AIH (A-AIH) or chronic AIH (C-AIH). Results: Acute onset of AIH occurred in 19/40 children (48％). Six of them fulfilled the criteria of ALF with coagulopathy and encephalopathy. Five of 6 children with ALF suffered from exacerbation of previ- ously undiagnosed chronic AIH, among which 4 children were histologically confirmed as micronodular cirrhosis. The remaining one patient had fulminant AIH with centrilobular necrosis, but no histological signs of previous chronic liver damage. We observed significantly lower levels of albumin, higher levels of aminotransferases, bilirubin, INR, IgG, higher IAIHG score and more severe histological findings in A- AIH than in C-AIH. No differences in patient age and presence of autoantibodies were observed between A-AIH and C-AIH. All children, including those with ALF and cirrhosis, were treated with corticosteroids, and are alive and achieved AIH remission. Liver transplant was not indicated in any patient. Conclusion: Rapid and accurate diagnosis of A-AIH may be difficult. However, timely start of immunosup- pressive therapy improves prognosis and decreases number of indicated liver transplantations in children with AIH.