A surgically treated case of left atrial myxoma is reported. A 66-year-old man with a history of cough and orthpnea had an echocardiographic and an MRI diagnosis of left atrial myxoma. He had the constitutional signs of myxoma including acceleration of E.S.R., positive CRP, hyperimmunoglobulinemia, loss of body weight, and so on, in addition to the symptoms of heart failure. Cardiac surgery was performed on him under extracorporeal circulation on June 12, 1990. A large myxoma with a diameter of 6.0 cm x 4.8 cm that was adhering to the fossa ovalis with a stalk was resected. Afterwards the symptoms of both heart failure and the constitutional signs disappeared, and the postoperative course was uneventful. Studies of the excised specimen demonstrated that this tumor produced Interleukin (IL-6). After operation the level of the serum IL-6 that was high before operation was normalized. This suggests that the symptoms and the laboratory results pointing to an autoimmune disease were due to the IL-6 produced from the cardiac myxoma. This is the first report that the localization of the IL-6 in the left atrial myxoma is demonstrated with immunohistochemical stain.