An 83-year-old woman, who had suffered from idiopathic thrombocytopenic purpura (ITP), was admitted to our hospital because of cardiac heart failure and chest pain. The platelet was 42 x 10(4) in microl. Echocardiography revealed moderate aortic stenosis and regurgitation and left ventricular dysfunction. Preoperatively, we tapered oral steroid and administered high-dose immunoglobulin intravenously. Intraoperatively, we found quadricuspid aortic valve and the rudimentary accessory cusp was located between the right coronary cusp and noncoronary cusp. Aortic valve replacement was performed with bioprosthetic valve. The postoperative course was uneventful. Postoperative echocardiography revealed no perivalvular leakage. Preoperative administration of high-dose immunoglobulin and intraoperative platelet transfusion is very effective to minimize hemorrhagic complication in patients with ITP. We herein report an extremely rare quadricuspid aortic valve complicated with ITP.