Autoimmune hepatitis (AIH) is thought to be associated with various genetic and immunological abnormalities. Concerning the pathogenesis of AIH, increasing attention has been paid to genome-wide association studies, toll-like receptors and Treg/Th17 balance. For Japanese patients with AIH, novel diagnostic guidelines have been proposed in view of the differential clinical features between Japanese and Caucasian patients. However, the diagnosis of some patients in acute hepatitis phase is not easy. Histologically, centrilobular necrosis without portal inflammation is particularly characteristic in the acute hepatitis phase. Some patients become resistant to steroid therapy and have a very poor prognosis once they progress to acute hepatic failure. Therefore, additional revision of the current diagnostic criteria, including severity grading, will be needed in the future.