The antiphospholipid syndrome (APS) is a systemic autoimmune disease with an unclear pathophysiology. It is characterized by a hyper-coagulation status with frequent thrombotic events and the occurrence of antiphospholipid antibodies like lupus anticoagulants, anti-cardiolipin antibodies and b2-glycoprotein I antibodies in human plasma. In this study we analyzed the expression of VCAM-1, ICAM-1, E-selectin and annexin V in primary cultures of HUVEC in response to 10% (v/v) serum of control patients (n=6), patients with systemic lupus erythematosus (SLE) and no APS (n=3), and APS patients (n=10) for 24 h. The mRNA levels of all genes were increased with APS serum (e.g. ICAM-1: 166±22%; P=0.023). Serum of patients with (SLE)/no APS caused a 1.4-fold higher ICAM-1 mRNA level (not significant). Western blot analysis showed a significant increase in protein expression of adhesion molecules VCAM-1 (357±97%; P=0.023) and ICAM-1 (260±49%; P=0.011) with APS serum. Soluble forms of adhesion molecules were quantified by ELISA. Only sVCAM-1 showed in APS patients an elevated plasma level (189±34%; P=0.045). From our experiments we conclude that the augmented expression of adhesion molecules is important for the APS pathophysiology.