A 61-year-old woman was referred to our department for evaluation of an incidental adrenal mass. An abdominal CT scan revealed a 4.1 cm right adrenal mass. The patient had been diagnosed with hypertension 7 years earlier and had taken antihypertensive medications intermittently. Her physical examination demonstrated a round face, central obesity, and mild hypertension. Serum catecholamines, renin, aldosterone, ACTH and 24-h urine-free cortisol, vanillylmandelic acid levels were within normal limits. However, serum cortisol level was markedly elevated and the circadian rhythm was disturbed. Successive low-dose and high-dose dexamethasone suppression tests were ordered for evaluation of a functioning adrenal incidentaloma. About 2 h after taking the second dose of 2 mg dexamethasone, she suddenly developed nausea and vomiting, palpitations, and anxiety with severe hypertension. On the same day, we measured serum catecholamines, which were markedly elevated. An elective laparoscopic right adrenalectomy was performed and pathologic examination confirmed the diagnosis of pheochromocytoma. One week after surgery, serum and urine catecholamine levels returned to normal. The patient has remained normotensive without any medications and clinically well. Patients with adrenal incidentalomas may have a functional mass that does not always manifest as a full symptomatic disease. During the investigation of adrenal incidentalomas, pheochromocytoma should ideally be ruled out before administering corticosteroids.