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OBJECTIVE: To determine the prevalence of HIV DNA and RNA and the morphologic localization of HIV in the oral cavity of HIV-seropositive subjects. DESIGN: A cross-sectional analysis of saliva, buccal scrapings and buccal biopsies from HIV-seropositive injecting drug users (IDUs). SUBJECTS AND METHODS: Whole saliva, buccal mucosal scrapings and buccal biopsies were obtained from HIV-seropositive and seronegative IDUs. Presence of HIV DNA and RNA was assessed by polymerase chain reaction (PCR) and reverse transcriptase PCR (RT-PCR). RT in situ PCR was used to detect HIV tat/rev RNA in buccal mucosal scrapings. Host-cell integrated HIV-proviral DNA in buccal biopsies was detected by in situ PCR. Presence of intact HIV viral particles in buccal scrapings was assessed by electron microscopy. RESULTS: HIV DNA was detected in 40% (18/45) and HIV RNA in 69.2% (25/36) of saliva samples from HIV-seropositive IDUs. Viral particles consistent with HIV were localized in inter-epithelial spaces by electron microscopy. RT in situ PCR revealed the presence of HIV tat/rev RNA in 36% (8/22) of the seropositive samples tested. CONCLUSIONS: Our results suggest that epithelial cells can be productively infected by HIV. Epithelial cells in buccal mucosa may acquire HIV in the basal layers through contact with submucosal HIV-positive lymphocytes and/or Langerhans' cells. HIV infection may also spread by inter-epithelial cell contact. As HIV infected cells mature they travel to more superficial layers and are shed into the oral cavity