Abstract: Objective To investigate the features of various blood-borne virus infection and co-infection in intravenous drug users(DU).The correlation of Tlymphocyte subsets and virus co-infection was exam fined.Methods A study population of 406 DU consisted of 383 males and 23 females.All subjects had no clinical manifestation of hepatitis HBV-DNA and HCV-RNA were identified by fluorescence quantitative polymerase chain reaction HBsAg,HBeAg,anti-HCV ,HDV-Ag,anti-HCV,anti-HIV,and HCM V-IgM were assayed by enzym e-linked immunosorbent assay(ELISA)and immunochrom atogmphic tests.The levels of cytokines of Thl and Th2 were measured by EL ISA and radioactive immune assay (RIA).Tlymphocyte subpopulation was detected by means of fluorescence immunoassay.The indices taken from 102 healthy persons served as control.Results The infection rate of the vims among DU was 36.45% for HBV,69.70 for HCV,47.3% for HIV,2.22% forHDV,1.97% for HGV,and 3.45% for HCMV,respectively.The co-infection rate of blood-borne virus was detected in 255 of 406 (61.81%)subjects Triple infection and fourfold infectionwere detected in the study.More than 80%(161/192) of subjects infected with HIV were co-infected with the other virus (HBV or HCV).In contrast,among the controls,the infection rate was 17.65% for HBV and 0% for other vimses Therewas a profound decrease in the proportion of CD4/CD8 and the percentage of CD3 and CD4,but no decrease in the percentage of CD8 The levels of PHA-induced cytokines(IFH-γ and 1-4)and the level of seaun 1-4 were obviously decreased in DU.On the other hand,the level of seaun 1-4 was increased The level of IFH-γand the percentage of CD4 were continuously decreased when the DU was infected with HIV or HIV co-infection DU with HIV and HBV co-infection was 15.1% and 51.72%.37.93% of the subjects were HBV-DNA positive and HBeAg positive when there was HIV and HBV co-infection Only 1.68% (2/119) of DU without HIV infection were HBV-DNA positive. Conclusion HCV,HBV and HIV infection were comm on in DU.The co-infection led to a high incidence of inpaired Thl cytokine levels and CD4 lymphyocyte DU with HIV and HBV/HCV co-infection showed both lower expression of Thl cytokine and enhancement of the Th2 response HIV may be a reason causing HBV rep lication through decreasing Thl function.