A normal serum calcium level is 8 to 10 mg/dL. The diagnosis of hypercalcemia (ie, levels 10.5 mg/dL or greater) should be confirmed with an albumin-adjusted or ionized calcium level. The two most common causes of hypercalcemia are hyperparathyroidism and malignancy. Drugs, notably lithium and thiazide diuretics, also can cause hypercalcemia. Patients with severe or symptomatic hypercalcemia should be treated initially with hydration to decrease calcium levels. The evaluation should include a parathyroid hormone (PTH) level. If the PTH level is low, cancer is a likely cause, particularly multiple myeloma, breast cancer, or lymphoma. If the PTH level is normal or elevated, hyperparathyroidism is the likely cause. Symptomatic patients with hyperparathyroidism and patients with certain clinical markers should be considered for surgery. For patients with mild disease, monitoring is an option. Hypocalcemia often is caused by vitamin D deficiency. Symptomatic patients and patients with calcium levels less than 7.6 mg/dL should be treated with intravenous calcium gluconate; concomitant magnesium deficiency should be addressed. There is no evidence that routine calcium and vitamin D supplementation reduces the risk of fractures, but studies have shown that vitamin D supplementation does decrease the number of falls in older adults at risk.