BACKGROUND: A nationwide survey of autoimmune hepatitis (AIH) was conducted in Japan, and the clinical features of patients with AIH were analyzed. METHODS: A total of 1,056 patients were enrolled through questionnaires sent to 153 hospitals and clinics with hepatology specialists. RESULTS: The clinical features of Japanese AIH were as follows: (1) most patients were middle-aged women, with ages peaking in the 60s; (2) serum immunoglobulin G (IgG) levels were high, but more than 30% of the patients had serum IgG levels less than 2,000 mg/dl; the cutoff level of IgG to distinguish between patients with AIH and those with chronic hepatitis resulting from hepatitis virus was approximately 2,000 mg/dl; (3) autoantibodies, particularly the antinuclear antibody, were frequently present; (4) most patients showed histological features of chronic hepatitis or liver cirrhosis, but 10.9% of the patients showed histological features of acute hepatitis; (5) approximately 75% of the patients were treated with corticosteroids and showed a favorable response to treatment, whereas ursodeoxycholic acid was administered to 60% of the patients. CONCLUSIONS: The clinical features of patients with AIH in Japan were clarified. The mean age at diagnosis was higher than that in previous studies. In addition, the number of patients with AIH having the histological features of acute hepatitis increased. Diagnosis of AIH does not preclude the presence of acute hepatitis and/or serum IgG levels less than 2,000 mg/dl.