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/=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