An ectopic hyperfunctioning parathyroid gland is a wellknown cause of failure of an initial surgical procedure for primary hyperparathyroidism. Primary hyperparathyroidism, which is usually asymptomatic, occasionally presents with a hyper-parathyroid crisis. Techniques for preoperative localization are of great importance to prevent or minimize negative exploration and the possible need for future additional exploratory surgery. Surgical resection is the only way of managing mediastinal parathyroid adenoma. Without removal of the involved parathyroid glands, severe hypercalcemia will progress to multisystem organ failure affecting the gastrointestinal, cardiac, renal and cerebral functions, with the mortality rate approaching 100%. We describe a patient with hyperparathyroidism that was caused by a large posterior mediastinal parathyroid adenoma.