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OBJECTIVES To investigate the prevalence and relative titre of TT virus (TTV) DNA, and to examine the relationship between the extent of TTV viraemia and the immune status among 144 patients with HIV infection; 178 age- and sex-matched healthy individuals were also studied. METHODS TTV DNA was detected quantitatively by two distinct polymerase chain reaction (PCR) methods [untranslated region (UTR) and N22]. UTR PCR detects all TTV genotypes, and N22 PCR can primarily detect four major TTV genotypes (1-4). RESULTS Using UTR PCR and N22 PCR, respectively, TTV DNA was detected significantly more frequently in HIV-infected patients than in controls (99 versus 91%, P < 0.001; 56 versus 27%, P < 0.0001), and the relative titre (10N/ml) was significantly higher in HIV-infected patients [4.5 +/- 1.2 (mean +/- SD) versus 3.1 +/- 0.9, P < 0.0001; 2.6 +/- 1.5 versus 1.5 +/- 0.9, P < 0.0001]. Age, sex, co-infection with hepatitis B or C virus, and risk factors for HIV transmission did not appear to be significant factors associated with the titre of TTV viraemia. However, the titre of TTV DNA was significantly higher in HIV-infected patients with AIDS (P < 0.0001), those with low CD4 T cell count (P < 0.0001), or those with high HIV viral loads (P = 0.0047). CONCLUSION TTV is highly prevalent and high-titred in HIV-infected patients. The TTV viral load may reflect the degree of immune status of these immunocompromised hosts.