A 34-year-old man with hypercalcemia had a sestamibi scan that showed increased uptake that appeared to correspond with a 7 mm density in the mediastinum, adjacent to the aortic arch, on chest CT. This lesion was thought to be consistent with an ectopic parathyroid adenoma. Prior to surgical exploration, the patient was injected intravenously with 12.3 millicuries of Tc99m-sestamibi. The lesion identified preoperatively did not show abnormal radionuclide uptake using the intraoperative gamma probe, and was subsequently determined to be a hyperplastic lymph node. However, abnormal activity was localized to the thymus gland, from which an ectopic parathyroid adenoma was successfully excised. The patient recovered without incident. The use of the intraoperative gamma probe was critical in identifying and resecting the ectopic parathyroid adenoma in this patient, and in general, may reduce surgical time and reduce the morbidity and/or complications associated with surgical exploration.