Myxoma is a benign growth constituting nearly 50% of all primary cardiac tumors. It is important because it can be abolished by surgical therapy and is usually fatal if unrecognized and untreated. Recently a wider use of echocardiogram as the screening test of valve lesions can be expected to increase the number of myxomas found preoperatively. We report a case of left atrial myxoma which was diagnosed by echocardiogram before surgery, and was successfully removed with the aid of extracorporeal circulation. The patient was a 24-year old woman who had suffered from mitral valvular symptoms for 2 months before admission. At operation, a tumor, measuring 5.2 x 4.3 x 4.7 cm, was extremely friable, villous gelatinous mass and it was removed from its origin near the closed fossa ovalis, including its stalk and a portion of the septum. The postoperative course was uneventful. The patient is clinically well and without symptoms of heart disease.