Antiphospholipid antibodies are associated with arterial and venous thrombosis, recurrent pregnancy loss, and thrombocytopenia. Although the antibodies have not been conclusively shown to be causal in thrombosis and miscarriage, they are useful laboratory markers for the antiphospholipid syndrome. The identification of the syndrome is clinically important because of the risk of recurrent thrombosis and the need for antithrombotic therapy in many cases. Diagnosis and treatment of antiphospholipid syndrome is difficult, however, because of the protean clinical manifestations and associations, limitations of existing laboratory tests for antiphospholipid antibodies, and the absence of evidence-based guidance on best management.