Hypercalcemia is frequently encountered in healthy outpatients. Reliable measurements of the mediators of hypercalcemia have improved diagnostic certainty about the etiology in most patients. Hyperparathyroidism is overwhelmingly the most common cause. Medical evaluation of the patient with hyperparathyroidism requires an understanding of the complications of the disorder and the associated syndromes. At present decreased bone mineral density and nephrolithiasis are the major sequelae of hyperparathyroidism. Most cases of primary hyperparathyroidism are sporadic; however, hereditary forms can occur in patients with the multiple endocrine neoplasia syndromes. Surgery is the only curative therapy. Results are excellent when an experienced endocrine surgeon performs parathyroid surgery.