Patients with confirmed or suspected primary hyperparathyroidism or complications therefrom (such as postoperative hypoparathyroidism) will be admitted for diagnosis and treatment. The principal diagnostic components are calcium in serum and urine and parathyroid hormone in serum. Patients with moderately to highly severe primary hyperparathyroidism will be treated. Treatment will be mainly by parathyroidectomy. Other options are medications or no intervention. Patients with a hyperparathyroid syndrome may be managed for their extraparathyroid features. Preoperative testing to localize parathyroid neoplasm(s) will be used usually and with more extended methods in cases with prior neck surgery. Preoperative tumor localization tests will be selected according to clinical indications from the following: ultrasound, technetium-thallium scan, computerized tomography, magnetic resonance imaging, fine needle aspiration for parathyroid hormone assay, selective arteriogram, selective venous catheterization for parathyroid hormone assay. Options for management of postoperative hypocalcemia include calcium, vitamin D analogs, parathyroid autografts and synthetic parathyroid hormone. Research specimens may consist of blood or tumors.