Depending upon the method of measurement, hypocalcemia occurs in 15% to 88% and hypercalcemia occurs in 15% of hospitalized patients. Ionized calcium should be measured in place of total serum calcium to avoid errors related to hypoalbuminemia, which is seen commonly in acutely ill patients. Symptomatic hypocalcemia requires prompt intravenous calcium administration. Symptomatic hypercalcemia (most often caused by hyperparathyroidism or malignancy) warrants aggressive intravenous hydration to correct volume depletion and, usually, additional therapy with diuretics, bisphosphonates, calcitonin, or corticosteroids. Identification and treatment of the underlying cause of the calcium derangement must be addressed after the acute electrolyte abnormality is stabilized.