Infection with hepatitis B virus leads to a wide spectrum of liver injury, including self-limited acute hepatitis, fulminant hepatitis and chronic hepatitis with progression to cirrhosis or acute exacerbation to liver failure, as well as, an asymptomatic chronic carrier state. To investigate why an extreme immunological attack occurs in fulminant hepatitis and severe exacerbation, the entire precore and core region were sequenced in 13 subjects. No significant change was noted in the 6 self-limited acute hepatitis patients. In contrast, clustering changes in a small segment of core codons #84 to #99 were found in all 7 fulminant and severe exacerbation patients. These data suggest that this mutation clustering region may play an important role in the pathogenesis, and such mutations appear to be related to severe liver damage.