To assess the validity of a scoring system developed by the International Autoimmune Hepatitis Group for the definite diagnosis of autoimmune hepatitis, 119 patients with autoimmune hepatitis by standard clinical criteria and 131 patients with other chronic liver diseases were evaluated. Each patient was graded on 35 items in 13 clinical categories. Ninety-seven patients diagnosed as having autoimmune hepatitis by conventional criteria (82%) had a definite diagnosis by the scoring system and 22 patients (18%) had a probable diagnosis. Of these patients, those with definite diagnoses had significant clinical differences from those with probable diagnoses. Only two patients with other chronic liver disease (2%) had scores sufficient for a definite diagnosis of autoimmune hepatitis. Probable diagnoses, however, were common in other conditions (33%). Failure to adequately downgrade for cholestatic features contributed to these uncertain diagnoses. Scoring for treatment response downgraded the diagnosis in 17 of 93 patients with a definite diagnosis before therapy (18%) and upgraded the diagnosis in 6 of 14 others (43%). We conclude that the scoring system is specific for the definite diagnosis of autoimmune hepatitis, and it complements standard clinical criteria by establishing the strength of the diagnosis and defining distinctive subgroups within the diagnostic category. Refinements are necessary to reduce the frequency of probable diagnoses. Diagnoses at presentation can be commonly modified by scoring treatment response.