Abstract: To study the effects of mutations in the core promoter (CP) and the precore/core gene of hepatitis B virus (HBV) on acute clinical exacerbation in chronic asymptomatic HBV carriers (AsC) and the severity of patients with chronic hepatitis B. CP and precore/core gene were sequenced directly from sera of four AsCs, twenty-seven patients with chronic hepatitis B and three patients with chronic severe hepatitis B, after amplication by the polymerase chain reaction. Clusters of mutations in CP region, a A-to-C mutation at necleotide (nt) 1726, a A-to-T mutation at nt 1727 and a C-to-G mutation at nt 1730, were observed in eight of eleven AsCs developing acute exacerbation, Moreover, the clustering mutations were found negative in the state of AsC and positive after acute exacerbation in a subject. Five cases of the patients with both the clustering mutations and the double mutations (a A-to-T mutation at nt 1762 and a G-to-A mutation at nt 1764) manifested sever hepatitis or active cirrhosis with high level of serum HBV DNA and HBeAg/anti-HBe exchange. The clustering mutations are associated with acute exacerbation in HBV AsCs, and co-existence in the clustering mutations and the double mutations is related to the severity of chronic hepatitis B.