The unusual causes of hypercalcemia have been reviewed. These disorders are rarely derived as the cause of hypercalcemia from the usual tests that one obtains in working up hypercalcemic patients (such as PTH level, phosphorus, urinary calcium). These diagnoses (particularly drug-related hypercalcemia) can be determined only from a careful history. The vast majority of hypercalcemic patients have disease secondary to cancer, hyperparathyroidism, or disorders of vitamin D metabolism. It should be noted that some hypercalcemic patients may have more than one disease. Therefore, before assuming that a hypercalcemic patient with Paget's disease, thiazide ingestion, immobilization, or so forth has hypercalcemia secondary to the primary disorder, hyperparathyroidism and cancer should also be considered. Similarly, serum calcium levels can normalize in some patients with mild hyperparathyroidism or bony metastases with mobilization and/or cessation of thiazide therapy.