OBJECTIVE To study the relationship between human herpesvirus 6 (HHV-6) and oral squamous cell carcinoma. METHODS The serum anti-HHV-6 antibody titers from oral squamous cell carcinoma patients and control subjects were detected by indirect immunofluorescence assay. HHV-6 DNA in peripheral blood mononuclear cells from oral squamous cell carcinoma patients and control subjects was amplified by PCR with primers from sequence of HHV-6 and the specificity was confirmed by Southern-blot hybridization with an internal probe oligonucleotide. An immunohistochemical staining using rabbit anti-HHV-6 antibody was used to detect HHV-6 antigen in oral tumor tissues from oral squamous cell carcinoma patients. RESULTS Significantly higher proportion of patients with oral carcinoma (16/16) had IgG antibody to HHV-6 in sera compared with those (12/16) in control subjects, and geometric mean titer of these two groups was 1:118 and 1:64 respectively (P less than 0.05). The detectable rate of HHV-6 DNA in peripheral blood mononuclear cells for the above groups was 10/16 and 6/16 respectively (P less than 0.05). HHV-6 antigens were positive in 9 out of 12 oral tumor cases and in only 2 out of 8 pericancerous tissues the difference between these two groups was also significant (P less than 0.05). CONCLUSIONS These results demonstrated the frequent presence of HHV-6 in oral squamous cell carcinoma, therefore, HHV-6 possibly play a role in the pathogenesis of oral squamous cell carcinoma.