Coagulopathy leading to arterial and venous thrombosis is sometimes seen in patients with systemic lupus erythematosus (SLE). It is caused by antiphospholipid antibodies reacting with negatively charged phospholipid constituents of membranes and coagulation factors. The patients may also suffer from recurrent fetal loss and neurological manifestations, and may have intermittent thrombocytopenia and low serum concentration of complement factor C4. Patients without evidence of SLE or other immunological disorders may also exhibit this antiphospholipid antibody-associated thrombotic tendency. The patients are often young women, and it is important to be aware of this kind of coagulopathy, because it may take a most severe course in some of the persons affected.