Ectopic parathyroid adenoma is a frequent cause of persistent or recurrent hyperparathyroidism after parathyroidectomy in patients with chronic renal failure on dialysis. An unusual anatomic localization of parathyroid adenoma may make the diagnosis and surgery difficult. In a 41-year-old woman with chronic renal failure, increased serum level of parathyroid hormone and symptoms of progressive renal osteodystrophy, mediastinal parathyroid adenoma was detected in the aorticopulmonary window by 99m Tc sesta MIBI scintigraphy and transmission computed tomography. Extirpation of adenoma, sized 3 x 2 cm, was performed through a left thoracotomy. Serum parathormone level returned to normal and the patient steadily recovered.