AIMS: To determine how chronic hepatitis D virus (HDV) infection affects intrahepatic hepatitis B virus (HBV) antigen expression. METHODS: Ninety eight liver biopsy specimens from 68 patients seropositive for total antibody to HDV were studied by immunohistochemistry, and the amount of HBV antigens was also quantified by radioimmunoassay in 12 patients and compared with 30 patients with chronic HBV infection. RESULTS: Forty nine of the 68 patients were positive for intrahepatic HDV antigen and only five were positive for HBV core antigen (HBcAg). HBV surface antigen (HBsAg) was present in 55 (80.9%) patients and was always cytoplasmic in distribution. Hepatic pre-S1 and pre-S2 expressions paralleled that of HBsAg, and were detected in 53 (77.9%) and 54 (79.4%) patients, respectively. There was no relation between the intrahepatic expression of HDV antigen and HBsAg/pre-S1/pre-S2. Follow up biopsy specimens in 25 patients showed either static or deteriorating histology while intrahepatic HDV antigen remained the same or fell. The patients with intrahepatic expression of HBcAg had either absent or noticeably decreased expression of HBcAg in their follow up biopsy specimens (median two years). In contrast, HBsAg/pre-S1/pre-S2 were the same or increased (p less than 0.001). Quantification of intrahepatic HBsAg in patients with chronic HDV infection (0.61 pg/hepatocyte, range: 0.05-1.08, n = 12) showed no difference with patients with chronic HBV infection alone (0.64 pg/hepatocyte, range: 0.02-1.02, n = 30, p = NS). CONCLUSION: These data indicate that chronic HDV infection suppresses intrahepatic expression of HBcAg but not HbsAg and pre-S antigens, suggesting a differential effect of chronic HDV infection on HBV gene expression.