Sections of formalin-fixed paraffin-embedded minor labial salivary glands from 23 patients with Sjögren's syndrome associated with rheumatoid arthritis (secondary Sjögren's syndrome: sSS) and from 11 patients with keratoconjunctivitis sicca (KCS) associated with rheumatoid arthritis but without proven SS were examined by in situ hybridization using the BamH1 V (W) fragment of Epstein Barr viral (EBV) DNA. Minor labial salivary glands from 7 healthy individuals were used as the control. EBV specific DNA sequences were detected in the nuclei of epithelial cells in 16 out of 23 cases with sSS and in 3 out of 11 patients with KCS. None of the 7 salivary gland biopsies from the control group showed a positive hybridization signal. Epithelial cells containing the EBV genome were more frequently detected in areas with tissue destruction and lymphoepithelial lesions. Our results provide evidence for an increased EBV infection load in patients with sSS in comparison with control subjects and suggest that this virus may play a role in the pathogenesis of sSS.