Primary aldosteronism, not a common cause of high blood pressure, is a syndrome which results from excessively secreted aldosterone from adrenal gland and it accounts for 0.05-2.2% of unselected hypertension. In this case the lesion was not visualized on routine abdominal computed tomographic scan due to its small size. Therefore the selective adrenal venous catherterization & venous sampling was done. As there is some difficulty of sampling from Rt. adrenal vein, the method of measuring aldosterone vs. cortisol ratio of Lt. adrenal vein and inferior vena cava was used to localize the aldosterone-producing adenoma. Clinical symptoms normalized and laboratory data returned to normal range after the surgical adrenalectomy (J Kor Soc Endocrinol 13:652-658, 1998).