In order to examine the clinical features of severe acute exacerbation in chronic hepatitis B virus (HBV) infection, 297 hepatitis B surface antigen (HBsAg) carriers were followed for 35+22 months (mean+s.D.) in Tohoku University Hospital from 1976 to 1987. Of these, 10 experienced severe acute exacerbation with hepatic decompensation. All of these patients had intense subjective symptoms related to the hepatitis. They were all icteric and 8 had ascites. Three developed to fulminant hepatic failure, eventually died. Histology after the exacerbation showed severe hepatic damage such as massive hepatic necrosis and bridging hepatic necrosis in a half of them. Six cases were suspected to result from spontaneous reactivation and 2 from drug-induced reactivation of chronic HBV infection, and the other 2 from superinfection with non-A, non-B hepatitis agent (s). These results suggest that the reactivation of chronic HBV infection is an important factor of severe acute exacerbations in chronic HBV infection in Japan.severe acute exacerbation ; HBsAg carrier ; reactivation of chronic HBV infection Patients with chronic type B hepatitis positive for hepatitis B e antigen (HBeAg) are considered to experience frequent acute exacerbations (Perrilo et al. 1984;Liaw et al. 1987). However, the exacerbations occasionally occur in hepatitis B surface antigen (HBsAg) carriers negative for HBeAg (Liaw et al. 1987). Such exacerbations are considered to be induced by the spontaneous reactivation of chronic hepatitis B virus (HBV) infection and superinfections with hepatitis A virus (HAY) (Zachoval et al. 1983;Davis et al. 1984b), non-A, non-B (NANB) hepatitis agent (s) (Papaevangelou et al. 1984) and hepatitis delta