Primary hyperparathyroidism is the most frequent cause of hypercalcemia in ambulatory patients. The condition is most common in postmenopausal women, although it can occur in persons of all ages, including pregnant women. If symptoms are present, they are attributable to hypercalcemia and may include weakness, easy fatigability, anorexia, or anxiety. However, most persons have no symptoms, and primary hyperparathyroidism usually is diagnosed after an elevated serum calcium level is found incidentally on multiphasic chemistry panel testing. Persistent hypercalcemia and an elevated serum parathyroid hormone level are the diagnostic criteria for primary hyperparathyroidism. Other causes of hypercalcemia are rare, and usually are associated with low (or sometimes normal) parathyroid hormone levels. Malignancy is the most frequent cause of hypercalcemia in hospitalized patients. Parathyroidectomy is the definitive treatment for primary hyperparathyroidism. When performed by experienced endocrine surgeons, the procedure has success rates of 90 to 95 percent and a low rate of complications. Asymptomatic patients who decline surgery and meet criteria for medical management must commit to conscientious long-term monitoring. Any unexplained elevation of the serum calcium level should be evaluated promptly to prevent complications from hypercalcemia.