The antiphospholipid antibodies (APA) are acquired antibodies against a phospholipid which has been associated with slow progressive thrombosis and infarction in the placenta. Clinical features (venous or arterial thrombosis, recurrent fetal loss, thrombocytopenia) in conjunction with positive laboratory findings (positive IgG or IgM anticardiolipin antibodies, or positive lupus anticoagulant tests) will satisfy criteria for diagnosis of the antiphospholipid antibody syndrome (APS). A number of studies report the incidence of antiphospholipid antibodies in different patient populations: normal obstetrical patients (5.3% of 7278 women), women with recurrent pregnancy loss (20% of 2226 women), women with systemic lupus erythematosus (37% of 1579 women) and, more recently, women undergoing in vitro fertilization (24% of 3343 women). As in all autoimmune syndromes it is possible that APA are secondary to some underlying disease or that they are instrumental in the pathogenesis of the various manifestations. The most commonly proposed mechanisms of antiphospholipid antibody induced thrombosis include decreased prostacycline production by endothelial cells, increased thromboxane production by platelets, and decreased protein C activation. More recently it has been demonstrated that certain phospholipids are exposed on the endothelial surface and may alter implantation during in vitro fertilization. Treatment with subcutaneous heparin and aspirin has been shown to benefit women with recurrent pregnancy loss and APA resulting in successfully deliveries of approximately 75%. Several trials of treatment with heparin and aspirin in women with positive APA undergoing IVF have been completed. Although none of the studies were randomized, prospective, blinded trials there does not appear to be a significant difference in implantation rate, pregnancy rate, or ongoing pregnancy rate. This subject remains, however, an area of active investigation as antiphospholipid antibodies have been shown to interact with syncytiotrophoblast and cytotrophoblast layers and could theoretically affect implantation.