Autoantibodies in chronic immune thrombocytopenic purpura occasionally interfere with platelet function. We describe a patient with a normal platelet count who had clinically significant mucosal bleeding, a prolonged bleeding time and abnormal platelet aggregation. The patient had high titres of an IgG4 kappa autoantibody, directed to a cation-dependent epitope on platelet glycoprotein IIb/IIIa, which blocked the binding of fibrinogen and fibronectin to this complex. Corticosteroid treatment resulted in clinical improvement and a marked drop in autoantibody concentration. The lack of thrombocytopenia in this patient, despite high autoantibody levels, is best explained by the poor recognition of IgG4 antibodies by phagocytic cells.