In order to clarify the clinical features of autoimmune hepatitis (AIH) without hypergammaglobulinemia (hyper-γ), we clinicopathologically evaluated 80 patients negative for markers of hepatotropic viruses and other causes of liver dysfunction who had probable or definite AIH according to the international criteria (AIH score). They were separated in two groups who had AIH with (≥2.0g/dl) or without (<2.0g/dl) hyper-γ. The patients without hyper-γ (5 males and 16 females, mean age : 41.8yr) were significantly younger than the patients with hyper-γ (8 males and 51 females, mean age : 53.2 yr), and had two peaks in age distribution (aged<30yr and≥60yr), and more commonly had acute-onset liver injury, hyperbilirubinemia, and raised serum ALT levels than patients with hyper-γ. Among patients without hyper-γ, young patients (aged<30yr) and elderly male patients (aged≥60 yr) showed a higher rate of acute onset. Seven patients without hyper-γ were treated with glucocorticoids and all achieved complete remission. Furthermore, two young patients without hyper-γ showed severe acute liver injury and a good response to glucocorticoids and azathioprine. These results suggest that many cases of AIH without hyper-γ are caused by an autoimmune mechanism, and that young patients without hyper-γ may be in the early stage of AIH. Since early treatments of young AIH patients achieved a good prognosis, the possibility of AIH should be considered in cases of severe hepatic injury of unknown etiology especially in young female patients.