To elucidate the precise mechanisms of the generation of chronic hepatitis B,we first examined the uncleotide sequences of HBV and found that the pre-S2 region of hepatitis B virus (HBV) was classified into three types according to their nucleotide sequences. Subsequently, we examined the pre-S2 type in 45HBV-infected chronic hepatitis B patients together with HLA class I phenotype and disease activity. The HLA-A24 antigen was found in 27 of 32 type I pre-S2 HBV-infected patients with active hepatitis, whereas of 6 patients with inactive disease, none had HLA-A24. Serial studies in 13 patients with HLA-A24 showed that with remission of hepatitis, clearance of type I pre-S2 HBV was seen in all patients. Furthermore, we were successful to establish the CTL clones that recognize the molecular complex composed by HLA-A24 and the pre-S2 peptide. These results suggest that HLA-A24 expression in the setting of type I pre-S2 HBV infection may play a role in the pathogenesis of chronic hepatitis B.