A 69-year-old woman suffered from bilateral renal stones. Laboratory examinations showed primary hyperparathyroidism. Neck exploration including left hemithyroidectomy and partial thymectomy failed to reveal the abnormal parathyroid gland. Localization studies were done for an ectopic parathyroid adenoma. Repeated selective arteriography with digital subtraction technique revealed an ectopic parathyroid adenoma in the upper mediastinum. Computed tomography, ultrasonotomography and selective venous sampling also confirmed this finding. Subsequently, mediastinal exploration with partial sternotomy was performed and the ectopic parathyroid adenoma was removed. We discuss the preoperative localization studies for ectopic parathyroid adenoma.