We report here a female patient with cyclic thrombocytopenia associated with antiplatelet autoantibodies. There was an inverse relationship between the level of platelet-associated IgG and platelet count. Bone marrow megakaryocytes were normal in number even during the thrombocytopenia. The binding of monoclonal antibodies (mAbs) against glycoprotein (GP) IIb/IIIa to patient platelets was significantly inhibited in the thrombocytopenic phase, while these mAbs normally bound to patient platelets obtained during the normal platelet count. Western blotting and mAb-specific immobilization of platelet antigens showed that both plasma autoantibody and the eluted IgG from the patient platelets bound to GPIIIa. These results suggest that the periodic production of antiplatelet autoantibody against GPIIIa caused cyclic destruction of platelets in this patient.