No clear association between human disease and TT virus (TTV) has been documented. A possible pathogenic role of TTV was investigated in patients infected with human immunodeficiency virus (HIV). TTV serum concentrations were estimated in 185 HIV-infected patients by dilution polymerase chain reaction. Of these, 149 (76%) were TTV-positive, compared with 18 (7%) of 252 Danish blood donors (P<. 001). Of the HIV-infected patients who were TTV-positive, 72 (51%) had high TTV viremia (>/=5 times the highest concentration observed among blood donors, i.e., >/=3.5x105 TTV/mL of serum). High TTV viremia was associated with decreased survival (P<.001; relative hazard [RH], 2.0). There was a correlation between lower CD4+ T cell counts and higher TTV titers (P<.01). In a Cox regression model, CD4+ T cell count (P<.001), age (P<.001), HIV viral load (P<.001), beta2 microglobulin (P<.02), and high TTV viremia (P<.01; RH, 1.9) were independent predictors of survival. TTV is suspected to be an opportunistic pathogen with an independent influence on HIV progression.