Antiphospholipid-antibodies such as the anticardiolipin-antibodies may lead to a biological false positive test for syphilis. Numerous clinical studies have shown an augmented incidence of both arterial and venous thrombosis in patients with increased serum levels of these antibodies. Most of the trials were case reports of patients with lupus erythematodes. The results of prospective studies on the importance of these antibodies in patients with thrombosis but without autoimmune disorders were, however, somewhat contradictory. In vitro, several explanations for the development of thrombosis by the antibodies exist: inhibition of prostacyclin, prekallikrein, thrombomodulin, interaction with coagulation factors and/or antithrombin III and damage of thrombocytes. In contrast, experimental studies on isolated blood vessels show that IgM-anticardiolipin-antibodies evoke endothelium-independent relaxations and therefore may prevent vasospasm and thrombosis.