The antiphospholipid syndrome is characterized clinically by fetal loss and thrombosis and serologically by the presence of autoantibodies to lipid-binding proteins. In a model of this procoagulant condition in which these antibodies are injected into pregnant mice, fetal loss was prevented by blocking of complement activation. Specifically, interaction of complement component 5a (C5a) with its receptor is necessary for thrombosis of placental vasculature (see the related article beginning on page 1644). Inhibition of complement activation may have a therapeutic role in this disease.