Background: It has been widely thought that heat shock protein might be involved in autoimmune disease mechanisms in humans. Objectives: The present study was performed to evaluate the recognition of Porphyromonas gingivalis heat shock protein 60 (hsp60) and human hsp60 by immune sera in P. gingivalis -infected periodontitis and atherosclerosis patients. Materials and methods: Mononuclear cells from atheroma lesions were stimulated with P. gingivalis hsp and sera from periodontitis or atherosclerosis patients were subjected to Western immunoblotting to P. gingivalis hsp or human hsp, respectively. Results: Western immunoblot analysis demonstrated the dual reactivity of anti-P. gingivalis antisera with P. gingivalis hsp and human hsp. We could also establish P. gingivalis hsp-specific T cell lines from the atheroma lesions, a mixture of CD4 and CD8 cells producing the cytokines characteristic of both Th1 and Th2 subsets. Conclusion: These observations suggest the modulating effect of P. gingivalis hsp60 in the immunopathogenesis of periodontitis and atherosclerosis.