Summary. Heterogeneity of the hepatitis B virus (HBV) core gene has been reported to be associated with the presence of active liver disease in Japanese patients with chronic HBV infection. This study evaluated the significance of HBV core gene heterogeneity in Western patients with chronic HBV infection. The hepatitis B virus precore/core gene from 45 patients (inactive:active liver disease ratio 16:29) was amplified from serum by polymerase chain reaction (PCR). Gel electrophoresis was employed to detect large deletions. The PCR amplicons from 13 patients (all HBV serotype adw but with a different spectrum of liver disease) were cloned and sequenced. Hepatitis B surface antigen (HBsAg) serotypes were tested by enzyme immunoassay (EIA) and hepatic expression of HBV antigens was assessed by immunohistochemistry. The HBV core gene was amplified from the serum of all 45 patients. Three patients had mixed infection with both precore mutant and wild-type HBV and all three had active liver disease. No patient had a large deletion of the HBV core gene. Hepatitis B virus core gene sequence variations were more common in the midcore region and there was no difference in the number of silent and missense substitutions between those with inactive and active liver disease. There was no correlation between the nucleotide or encoded amino acid substitutions and the clinical and biochemical parameters, including the subsequent response to interferon-α therapy (n= 37) or hepatic HBV antigen expression. Variation of the HBV core gene was not found to be preferentially associated with active liver disease in Western patients with chronic HBV infection. The pattern of hepatitis B core gene variation is in accord with the genomic organization of HBV.