Several studies have demonstrated the presence of serum HDV-RNA by molecular hybridization in patients with chronic D infection, but there is scarce information about the presence of HDV-RNA in hepatic tissue. The presence of HDV-RNA by in situ hybridization (ISH) with a non-radioactive probe in paraffin-embedded hepatic tissue was studied in 29 patients with chronic delta hepatitis (20 with and 9 without intrahepatic delta antigen) and correlate their presence with the expression of hepatic delta antigen and serum HDV-RNA by dot-blot hybridization. HDV-RNA was detected by in situ hybridization in 18 cases: 16 of the 20 (80%) biopsies with intrahepatic delta antigen and 2 of the 9 (22%) without. HDV-RNA was not detected in any of the control cases. Serum HDV-RNA was found in 19 cases: 18 (90%) of 20 chronic delta hepatitis cases with tissular delta antigen and one of the 9 without intrahepatic delta antigen. All patients except one, who was positive for intrahepatic HDV-RNA, showed serum HDV-RNA. However, in two cases ISH detected hepatic HDV-RNA without tissular HDAg; one of these also had serum HDV-RNA and in the other positivity for hepatic HDV-RNA by ISH was the only marker of viral delta replication. In conclusion, detection of HDV-RNA in hepatic tissue by in situ hybridization with a digoxygenin-labelled probe is a rapid and sensitive method that could be a useful tool for diagnosis of HDV infection in clinical laboratories.