The etiology, pathophysiology, and diagnosis of hypercalcemia associated with malignant diseases are discussed. In humans, calcium is controlled by three mechanisms: parathyroid hormone, which regulates bone resorption and renal reabsorption of calcium; calcitonin, an antagonist of parathyroid hormone; and cholecalciferol, which regulates calcium absorption from the gastrointestinal tract. Hypercalcemia of malignancy (HCM) results primarily from increased bone resorption by osteoclasts and, to a lesser extent, from increased renal tubular reabsorption. In most tumors, parathyroid hormone-related protein (PTHrP) is the primary mediator of calcium. PTHrP stimulates increased bone resorption by osteoclasts. This stimulation also activates transforming growth factor-beta (TGF-beta), which stimulates tumor cells, thus perpetuating the cycle. Hypercalcemia is usually defined as a serum calcium concentration greater than 12 mg/dL, corrected for the serum albumin concentration. In diagnosing HCM, it is important to rule out other causes of hypercalcemia, such as primary hyperparathyroidism.