The causes of acute clinical exacerbations, and the role of reactivation of hepatitis B virus (HBV) in 16 non-cirrhotic patients with chronic active type B hepatitis (CAH-B) negative for serum hepatitis B e antigen (HBeAg) but positive for anti-HBE, were studied by molecular hybridization and immunohistochemical methods. IgM antibody to hepatitis A virus (anti-HAV IgM) and antibody to delta agent (anti-delta) were negative in all. HBeAg reappeared transiently in only two patients. Serum hepatitis B virus (HBV) DNA levels increased during acute exacerbations in 14 patients (88%), and decreased after the episode. Cytoplasmic hepatitis B core antigen (HBcAg) expression was found in 9 out of 13 patients (69%) during acute exacerbation. By Southern blot hybridization, 5 of 6 (83%) liver tissues obtained during clinical exacerbations had free replicative forms of HBV DNA. In 20 control patients with no exacerbation, serum HBV DNA, HBcAg expression in hepatocytes and free replicative forms of HBV DNA were positive in 15% (3/20), 10% (2/20) and 25% (2/8), respectively--figures significantly lower than those of the group studied. We conclude that acute exacerbations sometimes seen in patients with anti-HBe-positive CAH-B in Taiwan are caused mainly by reactivation of HBV.