We report a case of unilateral hyperplasia of the adrenal medulla. The patient showed clinical features suggestive of pheochromocytoma. Removal of the hyperplastic adrenal gland resulted in complete disappearance of all prior symptoms, decrease of the plasma and urinary catecolamine levels and no high uptake in [133I] metaiodobenzylguanidine scintigraphy. A histological study revealed diffuse hyperplasia of the adrenal medulla. Up to now, there are relatively few reports of adrenal medullary hyperplasia in English literatures.