A 72-year-old Japanese woman with pheochromocytoma, who had had no characteristic symptoms was treated. A large retroperitoneal tumor was discovered incidentally by sonographic examination for mild upper abdominal pain and, with CT-scan and abdominal angiography confirmed that the tumor originated in the right adrenal gland. The tumor was suspected of being a pheochromocytoma because preoperative laboratory examinations revealed only a mild elevation of daily urinary excretions of adrenaline and noradrenaline. Provocation tests for pheochromocytoma and even angiographic examination revealed no diagnostic change in serum levels of catecholamines and distinctive clinical signs were nil. Thus, surgery was performed without preoperative prescription of any catecholamine blockade. During the surgery, the blood pressure and pulse rate fluctuated considerably. A non-functioning pheochromocytoma detected incidentally must be preoperatively managed as a functioning one, even in the absence of specific symptoms of pheochromocytoma.