In this retrospective study, we investigated the prevalence and significance of mutations in part of the hepatitis B virus (HBV) x gene, and tried to clarify their relationship with clinicopathological or histopathological characteristics and prognosis in patients with chronic hepatitis B (CHB). A total of 83 consecutive CHB patients (1986-1994) were chosen for the present study. Sequence analysis was performed using polymerase chain reaction (PCR) and the direct sequencing method. The histological activity index was described using Scheuer scores. Two-step immunohistochemical staining showed the expression of viral antigens in situ. Tissue HBV DNA levels were determined by fluorescence quantitative real-time PCR. For the prognostic study, all the patients were followed up using clinical and laboratory data. Mutation at nt1726-1730 correlated significantly with decreased expression of HBcAg in situ (P = 0.006) and with lower HBV DNA levels in the liver (P = 0.004). In particular, the CTGAC mutation showed the strongest decrease of the viral load (P = 0.007). By contrast, nt1762/1764 mutation correlated with increased HBcAg (P = 0.005) and higher HBV DNA levels (P = 0.006). The mutants with the wild-type of nt1726-1730 or nt1762/1764 mutation were more prevalent in hepatocellular carcinoma (HCC) patients than in CHB patients. Although the mutations did not correlate with cirrhosis, the frequency of nt1762/1764 mutation in patients with hepatocarcinogenesis was significantly higher than in those without hepatocarcinogenesis (P = 0.011). Mutations at nt1726-1730 and nt1762/1764 are associated with in situ expression of HBcAg and viral load. Higher HBV DNA levels in the liver may be associated with hepatocarcinogenesis. Mutation at nt1762/1764 remarkably increases the risk of hepatocarcinogenesis.