99mTc sestamibi parathyroid scan was performed in a 35-year-old man with chronic renal failure to evaluate the cause of increased serum parathyroid hormone. Both early and delayed images showed intense activity overlapping the left upper pole of the thyroid, which was proven to be nodular parathyroid hyperplasia after parathyroidectomy. However, the levels of serum parathyroid hormone remained elevated postsurgically. A repeat sestamibi study revealed abnormal activity in the left upper mediastinum, which could also be subtly seen on initial sestamibi study. A retrosternal lesion was resected, which was confirmed as an ectopic parathyroid adenoma by pathological examination.