ObjectiveTo evaluate the diagnostic value of immunohistochemistry(IHC) for the identification and localization of Treponema pallidum(TP) in secondary syphilitic lesions.MethodsSkin tissue specimens were obtained from the lesions of 25 patients with secondary syphilis and 15 patients with dermatoses unrelated to TP infection,followed by fixation and embeding.IHC using a polyclonal antibody against TP and Warthin-Starry (W-S) silver stain were carried out respectively to detect TP in these specimens.Results TP was detected in 80.00％ (20/25) of the specimens by IHC,44.00％ by W-S silver stain (Fisher's Exact Test,P =0.046).Of the 20 IHC-based TP-positive specimens,all harbored TP in the epidermis,11 also in the dermis.The density of TP was associated with the types of skin lesions,and sequentially decreased from condyloma latum to papules,maculopapules and maculae(x2 =15.694,P =0.011 ).Spirochetes were not seen in any of the control lesional specimens.ConclusionsIHC is superior to traditional W-S silver stain for detecting spirochetes in secondary syphilitic lesions,and is of great value to the diagnosis of secondary syphilis.The accurate localization of TP by IHC may facilitate the study on the formation of syphilitic lesions.