We report a case of left atrial myxoma found when examination was made for the cause of orthostatic hypotension. The case was that of a man of 61 years of age. For the previous 2 years, the man had felt dizzy only at the standing or sitting position. The blood pressure was 90/50 at recumbency and 64/40 at the sitting position. Echocardiographic study revealed a left atrial tumor, which fell into the left ventricular cavity and prevented the blood from filling the left ventricular cavity. This effect was more severe at the sitting position than at recumbency. Resection of the tumor was carried out. It was a myxoma with a diameter of 3.5 cm with a stalk adhering to the postero-inferior wall of the left atrium. After the removal of the tumor, the patient's complaint and orthostatic hypotension disappeared; blood pressure was 102/60 at recumbency and 98/64 at sitting position. Orthostatic dizziness has been reported in some cases as one of the symptoms of the intracardiac tumor. But the state of aggravation at the sitting position has never been observed during actual echocardiographic study. The myxoma adhered to the postero-inferior wall of the left atrium, which site might be associated with the symptom (orthostatic hypotension).