The clinical and biochemical features of patients with autoimmune hepatitis (AIH) in Japan during the 15-year period, 1975 to 1990, were evaluated by a nationwide survey using a questionnaire prepared by the Intractable Diseases Study Group organized by the Ministry of Health and Welfare. Data on a total of 866 cases with AIH were collected and analyzed, and of these, 632 cases underwent follow-up study. AIH was predominant in females, with a male:female ratio of approximately 2:19. Biochemical investigation on the occasion of the first examination showed abnormalities of liver function tests, including increased serum total bilirubin, transaminase activity and gamma-globulin levels. The main complications of autoimmune diseases were rheumatoid arthritis, Sjögren's syndrome and chronic thyroiditis. All patients with complications were female. Of 253 AIH patients, 50 were positive for anti-C100-3 antibody. ICG clearance and platelet count, which are stigmata of liver cirrhosis, were the main factors of prognostic importance in AIH. Therefore, early diagnosis prior to the development of liver cirrhosis is important to improve the prognosis of AIH.