Of 365 patients surgically treated for hyperparathyroidism at the University of Toronto hospitals, 3 had hypercalcemia due to an abnormal and ectopic fifth parathyroid gland. Autopsy studies have shown that a fifth gland may be present in 3% to 5% of patients with hyperparathyroidism. The possibility of an abnormal fifth gland as the cause of primary hyperparathyroidism should be considered when four glands of normal size and histology have been found in the neck, and such a gland should be sought in all patients with the diffuse hyperplasia of secondary hyperparathyroidism. The fifth gland is usually in the lower neck or upper mediastinum, frequently within the thymus. If present, it can usually be recognized and excised.