It has become increasingly appreciated that, although rare, a supernumerary (fifth) parathyroid gland may be a cause of excess parathyroid hormone production. This review reports 15 patients, each of whom had hyperparathyroidism as a consequence of such a hyperfunctioning fifth gland. It is apparent that these patients are similar to the usual patients with primary hyperparathyroidism with respect to complications of the disease, type of pathology, and care after surgical excision of the abnormal parathyroid tissue. Significantly different, however, is the fact that the majority of these fifth-gland tumors were located in the mediastinum. The circumstances under which a supernumerary gland should be expected and the recommended surgical approach are reviewed.