This is a report of a case in which a left atrial myxoma was successfully removed by total cardiopulmonary bypass with total hemodilution and moderate hypothermia. The patient was a 17 old girl with mitral valvular symptoms that occured suddenly three months prior to operation. After the onset of symptoms. it was progressively aggravated to serious general condition. The preoperative diagnosis of left atrial tumor was made from echocardiography. and the tumor was confirmed as myxoma by the histopathological examination of the tumor tissue removed during operation. The myxoma was unusually originated from posterior wall of the left atrium. which was located between drainage orifices of right and left pulmonary veins, The tumor developed in the left atrial cavity and one of the polypoid processes of the myxoma was prolapsed into the the left ventricle through the mitral orifice. Following operation. there was no evidence for peripheral vascular embolism, and all valvular symptoms disappeared. The postoperative course was uneventful.