The antiphospholipid antibody syndrome (APS) is defined by widespread arterial and venous thromboses associated with elevated plasma levels of antiphospholipid antibodies (APLA). The primary antiphospholipid antibody syndrome (PAPS) appear to be a fairly homogeneous disease, and HLA, family and other studies provide new insights into this cause of thrombosis and vascular disease. We describe two patients with PAPS (lupus anticoagulant positive), whose family members were analyzed for clinical and laboratory abnormalities associated with APS. Familial screening seems to be important, in order to prevent the thrombotic events. Low dose aspirin is the first line treatment in asymptomatic subjects with APLA, previous or present thrombosis requiring long-term, possibly life-long anticoagulation.