Much has happened since Conley and Hartmann [1] described the lupus inhibitor in two SLE patients in 1952. These patients had prolonged coagulation tests and biologically false positive syphilis tests. Although the term ‘‘lupus anticoagulant’’ (LA) was coined for this inhibitor [2], it soon became clear that it is neither confined to patients with SLE nor a coagulation inhibitor in vivo. The paradoxical association with thrombosis was suspected in the early 1960s [3]. Thus, the laboratory diagnosis of LA became important. In 1980, it was reported that the LA activity of a plasma was caused by antibodies against negatively charged phospholipids (PL) [4]. In the early 1980s, several groups tried to simplify the laboratory diagnosis using solid phase PL as antigens [5]. As the main antigen in the serological tests for syphilis is cardiolipin (CL), CL was selected as an antigen in these assays [6], although this PL is associated with mitochondrial rather than cell membranes and does not play a part in coagulation in vivo. Further work demonstrated that the concordance between coagulation tests for LA and the anticardiolipin (aCL) assay was only about 60% [7]. However, since patients negative for LA, but positive for aCL, seemed to have similar clinical symptoms, aCL ELISA tests became established as a routine investigation in addition to coagulation tests for LA. Around 1990, two independent groups showed that the real antigen in the anti-cardiolipin test was a glycoprotein, h2-glycoprotein I (h2GPI), rather than CL itself [8,9]. When the protein binds to a negatively charged surface, antigenic sites are exposed. A similar surface may be provided by microtiter plates. Thus, some ‘‘antiphospholipid antibodies’’ (APA) bind directly to h2GPI in the absence of PL. Some LA antibodies react with proteins other than h2GPI; most frequently prothrombin [10]. Conversely, some antibodies to h2GPI may be devoid of LA activity [11]. In general, the requirement for a protein cofactor distinguishes patients with the antiphospholipid syndrome (APS) from patients with syphilis and other infections. It has now been shown that autoimmune APA may be directed against a variety of PL-binding proteins. The most important of these protein cofactors, next to h2GPI and prothrombin, are protein C, protein S, annexin V, HMW kininogen, and factor XI. Individual patients often have a mixture of antibodies with different specificities. At present, the laboratory aspects of the APS are rather complicated (Fig. 1). The ability of some APA to prolong coagulation in vitro seems to depend on the formation of bivalent complexes between antibodies and either h2GPI [12] or prothrombin [13]. These complexes have increased affinity for PL and compete with coagulation factors for the same catalytic surface. The pathogenic mechanisms of APA have been difficult to unravel. Some of the hypotheses that have been proposed are activation of endothelial cells, blood platelets or monocytes, acquired protein C resistance, and impaired fibrinolytic capacity (for a review, see Ref. [14]. Actually, there is so far no direct evidence in humans that these antibodies are pathogenic rather than just laboratory markers. However, in experimental animals, APA may cause at least some of the disease states associated with the APS [15]. The diverse antibody specificities found in patients with the APS indicate that different pathogenic mechanisms may be involved. In addition to venous and arterial thrombosis, thrombocytopenia and recurrent pregnancy loss, a large variety of clinical symptoms are found to be associated with APA. Classification criteria for the APS were recently redefined [16] and have been validated [17,18] (Table 1). The delineation of the APS is a challenge, because the clinical features are common in other settings and the laboratory tests are not specific for this syndrome. Regardless of whether APA are pathogenic or simply disease markers, mounting clinical evidence suggests that they are important risk factors. In SLE (secondary APS), aCL or LA positivity defines a group with a substantially increased risk of pregnancy complications and venous and arterial thrombosis [19–21]. In individuals with no underlying autoimmune disease, APA positivity implies an