It may sometimes be difficult to distinguish primary from secondary hyperparathyroidism when advanced renal failure coexists. We report here the case of a patient with end-stage renal failure who had severe hyperparathyroidism. Cervical exploration revealed only the presence of four parathyroid glands normal in size and histological appearance which were removed. Because the existence of severe hyperparathyroidism had been firmly established based on biochemical and radiological evidence, the diagnosis of primary hyperparathyroidism due to an ectopic adenoma became obvious. Digital angiography and computerized tomography were then carried out. The results of angiography were inconclusive but computerized tomography revealed and precisely localized a mediastinal adenoma which was subsequently removed via sternotomy. The existence of a hypoparathyroid state was confirmed over the following two months. Reimplantation of parathyroid fragments which had been cryopreserved during the first operation, was then performed with success.