We have investigated whether or not a relationship exists between anti-cardiolipin antibody (aCL) positivity and human immunodeficiency virus type 1 (HIV) and/or hepatitis C virus (HCV), and we have attempted to clarify which virus has close association with the development of aCL. We found that aCL positivity in HIV-infected patients was significantly higher than in HCV-infected patients. Furthermore, HIV/HCV dual-infected patients exhibited a higher aCL positivity than patients infected by HCV alone. From these results, we conclude that HIV rather than HCV plays an important role in the development of aCL.