Hypercalcemia a frequent electrolyte disturbance, which is most commonly caused by cancer and parathyroid hormone (PTH) disturbances. The mechanisms by which cancer produces hypercalcemia are: (1) direct bone involvement, and (2) the production of substances that may accelerate bone resorption. In addition to PTH-like material, two such substances are prostaglandins and the so-called osteoclast activating factor, which is a material isolated from malignant cell lines, particularly lymphomas and multiple myeloma.1-3 Excessive secretion of PTH by parathyroid adenomas, hyperplasia, or carcinoma causes hypercalcemia by bone resorption, a mechanism potentiated by the active metabolite of vitamin D, 1,25-dihydroxycholecalciferol.4-5 Excessive formation or enhanced sensitivity to this compound, which in addition to potentiating PTH, increases gastrointestinal calcium reabsorption (as in sarcoidosis), is probably also a common cause of hypercalcemia. It is not within the scope of the present review to provide an extensive list of causes of hypercalcemia. Rather, we emphasize that