We examined the clinical characteristics of 89 patients with autoimmune hepatitis (AIH) in Nagasaki Prefecture, Japan, and assessed the usefulness of a provisional scoring system for the diagnosis of AIH proposed by the International Autoimmune Hepatitis Group in 1993. The majority of patients were middle-aged women in their fifties. All patients showed a hepatitic picture. Forty-three patients (48%) had an insidious or chronic onset, while 34 (38%) had an acute onset, and 12 (14%) had liver cirrhosis at presentation. Seventy-nine patients (89%) were positive for antinuclear antibody (ANA), and 5 (6%) were positive for antibody to the hepatitis C virus (anti-HCV). The prognosis was good, with 90% 3-year survival, and most patients responded well to treatment with corticosteroids. The international scoring system was useful for the diagnosis of AIH in most of our patients; the percentages of patients with definite and probable AIH were 48% and 47%, respectively. However, certain factors, such as negative ANA, positive antimitochondrial antibody, concurrent infection with hepatitis B or C virus, and insufficient response to treatment precluded the diagnosis of AIH in some patients. Whether these patients were indeed "true" AIH patients is not clear at present, and further investigation of such patients may be useful for a better understanding of AIH.