['The diagnosis of acute-onset autoimmune hepatitis (AIH) has been difficult because patients do not always show clinicopathological features typical of AIH. We examined the important requirements for a definitive diagnosis of acute-onset AIH.', 'Clinical, biochemical, and pathological features of 18 acute-onset AIH patients (16 women, 2 men; mean age, 54.3 +/- 12.3 years) with no history of liver disease and no signs of chronicity were examined.', 'Mean ALT was 679 +/- 431 IU/l, and mean T-Bil was 2.4 +/- 2.9 mg/dl. Mean IgG was 1801 +/- 446 mg/dl, with 7 patients (39%) showing normal levels. Antinuclear antibody was <or=1: 40 in 7 (39%). Liver histology showed severe activity in 17 (94%) of the patients and severe acute hepatitis in 7 (39%). Centrizonal necrosis and plasma cell accumulation were characteristic for acute-onset AIH. AIH score ranged from 7 to 18 (13.2 +/- 3.8) before treatment. All patients were diagnosed and treated early and responded completely to therapy.'] Is precise histological evaluation of liver biopsy specimen indispensable for diagnosis and treatment of acute-onset autoimmune hepatitis?