Pheochromocytoma manifesting hypertensive episodes, is a catecholamine-secreting neoplasm, that arises from the chromaffin cells This clinical study was performed on 12 cases of pheocromocytoma treated surgically at the Department of Surgery of Chonnaom National University from January 1988 to June 1995. The results are as follows: 1) The male to female ratio was 1:2. The prevalent ages were the fifth decades (33%) and third, forth decades (25%). 2) The most frequent clinical symptoms and signs were hypertention and palpitation (83.3%), followed by headache, dyspnea, faintness and chest discomfort. 3) 41.7% of the patients presented clinical symptoms and signs less than 1 year of duration. 4) The 24-hour urine VMA was elevated in 100% of 10 cases. metanephrine was elevated in 77.8% of 9 cases. 5) Preoperative localization of tumor was easily acchieved by CT, US, MRI, 131-I MIBG scan, and angiography. 6) The alpha-adrenergic blocker was used preoperatively in 9 cases to control hypertension. 7) The tumor was located in the left adrenal gland in 6 cases (50%). right adrenal gland in 5 cases(41.7%), and the abdominal paraganglia in 1 case (8.3%). 8) In pathologic and clinical findings, 4 cases were malignant. Among them, 2 were recurred, mass excision was impossable in 1 case. 9) post operative blood pressure was returned to normotensive level in 70%, post operative 24- hour urine VMA and metanephrine were found to be within normal levels and significantly decreased in the major of cases. In conclusion, CT,US and MRI have good diagnostic results in preoperative localization of pheochromocytoma.and it is safe to operate pheochromocytoma with compatible preoperative preparation. Also, it is important to closely follow up with recurring or metastasis in pathologically proven malignant pheochromocytoma.