Two cases of left atrial myxoma and concomitant, but unrelated mitral valve disease are reported. The first patient had severe mitral stenosis and mitral regurgitation due to rheumatic heart disease in addition to a large immobile left atrial myxoma. At surgery, the tumor was considered unresectable because of its size and widespread attachment to the left atrium. A mitral commissurotomy was performed with temporary improvement. The second patient had mitral regurgitation due to mitral valve prolapse (myxomatous degeneration) and an unrelated tumor. The patient's condition improved after mitral valve reconstruction and resection of the tumor. In both cases, the relatively immobile tumor was not physically in contact with the mitral valve and probably was not responsible for the valve disease. Both patients presented with dyspnea, to which the tumors may have contributed by causing partial obstruction of the pulmonary veins. Diagnostic and therapeutic problems resulting from the association of atrial myxoma and unrelated mitral valve disease are discussed.