It has been clarified that hepatitis G/GB virus-C is the minor cause of acute and chronic non-A-E hepatitis. But there exist non-A-E viral hepatitis patients. Recently, one of non-A-E hepatitis associated virus was identified and the new virus was named TT virus. We highly detected TT virus in the serum of hepatitis patients. TT virus were reported to be detected 1-37% of the general population in the world. TT virus may account for only a minor part of acute non-A-E hepatitis in Japan. However, whether TT virus infection really causes severe acute hepatitis is required to be elucidated.