Antiphospholipid antibodies (aPL) have been reported to occur in numerous viral infections. We report a 24-year-old Japanese woman, who developed multiple venous thrombosis associated with the elevation of anticardiolipin IgM after acute viral infection presenting a mononucleosis-like illness. Two months later, aPL and thromboses disappeared. In this case both parvovirus B19 and cytomegalovirus antibodies IgM were elevated, which indicated the possibility of cross-reaction.