The aim of this study was to investigate the influence of opiate abuse on the expression of Toll-like receptor 9 (TLR9) in the peripheral blood mononuclear cells (PBMCs) of HIV-1-infected patients and to elucidate possible mechanisms involved in the enhancement of HIV-1 replication by opiate abuse. A total of 200 participants were enrolled in the study by random selection from methadone treatment centers and voluntary HIV counseling and testing centers in the cities of Nanning, Liuzhou, and Qinzhou. These participants included 50 HIV-positive opiate abusers (Opiates HIV(+) group), 50 HIV-negative opiate abusers (Opiates HIV(-) group), 50 HIV-positive subjects who were not opiate abusers (Non-opiates HIV (+) group), and 50 HIV-negative subjects who were not opiate abusers (Control group). PBMCs were isolated from the peripheral blood samples from the subjects and the expression levels of TLR9 mRNA and protein were determined by q-PCR and western blot respectively. There was no significant difference among the four groups in age, gender, nationality, domicile, marital status, educational background or duration of drug abuse (P > 0.05). The median viral loads of the Opiates HIV(+) were significantly higher than those of the Non-Opiates HIV(+) groups (4.450 x 10(3) and 3.977 x 10(3) copies/mL respectively, P < 0.05). The relative expression levels of TLR9 mRNA in the Opiates HIV(+), Non-Opiates HIV(+), Opiates HIV(-) and Control groups were (2.13 +/- 1.59) x 10(-3), (3.66 +/- 2.22) x 10(-3), (1.96 +/- 1.42) x 10(-3) and (7.66 +/- 4.87) x 10(-3), respectively. The expression of TLR9 mRNA was significantly lower in both HIV-1-infected and -uninfected groups of opiate abusers compared with groups of non-abusers (P < 0.05). There was no significant difference in TLR9 mRNA expression levels between the Opiates HIV(+) group and the Opiates HIV(-) group (P > 0.05). However, in the non-opiate groups, the expression levels of TLR9 mRNA in the HIV(+) group were significantly lower than that of the control group (P< 0.05). Western blot results confirmed that the expression of TLR9 protein was lower in the Opiates HIV(+), Non-Opiates HIV(+), and Opiates HIV(-) groups compared to the control group. These results suggest that opiate abuse can decrease the expression of TLR9 in PBMCs, which may result in the enhancement of HIV-1 infection and replication due to a decline in immune response mediated by the TLR9 pathway.