Severe hypercalcemia is a life-threatening medical emergency. It is most commonly caused by malignant tumors, but can also be caused by primary hyperparathyroidism or less often by a dysregulated production of active vitamin D in granulomatous disorders. Symptoms include nausea, vomiting, renal insufficiency, severe dehydration, lethargy, confusion, and even coma. Severity of symptoms, calcium concentrations, and the overall status of the patient are important considerations in selecting appropriate therapy. Hydration to correct volume depletion is the cornerstone of acute therapy. Loop diuretics may be added to saline hydration after extracellular fluid volume has been replenished to enhance urinary calcium excretion and mitigate fluid overload from rehydration. Calcitonin and intravenous infusion of bisphosphonates reduce serum calcium levels by interfering with calcium release from the skeleton. Dialysis with a low or zero calcium dialysate is reserved for patients who are refractory to these measures. Corticosteroids are effective with hypercalcemia due to increased vitamin D levels and in multiple myeloma.