AIMS: To prospectively study whether occult hepatitis B virus (HBV) infection can promote the development of hepatocellular carcinoma (HCC) in patients with hepatitis C virus (HCV)-related chronic liver disease. In addition, to evaluate the difference among HBV DNA-negative patients and patients with high and low HBV copy numbers. METHODS: A total of 167 patients with HCV-related chronic liver disease without HBV surface antigen (HBsAg) were studied. HBV DNA in liver tissue was determined using polymerase chain reaction (PCR). RESULTS: HBV DNA was detected in 9 of 167 patients (5.4%) by single PCR and in 25 patients (15.0%) by nested PCR. HCC developed in 12 of 167 patients (7.2%). Ten of 142 HBV DNA-negative patients (7.0%) and 2 of 9 patients with a high HBV copy number (22.2%) developed HCC, whereas none of 16 patients with a low HBV copy number developed HCC. The incidence rate of HCC in patients with a high HBV copy number was significantly higher than in HBV DNA-negative patients and patients with low HBV copy number. CONCLUSION: A high amount of HBV DNA in liver tissue of HBsAg-negative patients with HCV-related liver disease might be associated with HCC development. Copyright (c) 2008 S. Karger AG, Basel.