Hypercalcemia is associated with a few primary malignant neoplasms and with a variety of tumors that have spread by metastases. Hyperparathyroidism is a diagnosis that is usually not considered in these patients. At our institution, 18 patients with malignant tumors presented over a 6-year period with hypercalcemia caused by hyperparathyroidism. There were five men and 13 women with a mean age of 48 years (range 24-87 years). Primary tumors in these patients included colon carcinoma (four cases), breast carcinoma (four cases), lymphoma (four cases), thyroid carcinoma (four cases), Paget's disease (one case), and lung carcinoma (one case). Metastases of the primary tumor occurred in seven patients, and in 11 patients the tumor was not metastatic or recurrent. Serum levels of calcium, phosphate, and chloride averaged 11.8 mg/dl, and 100 mEq/liter, respectively. C-terminal parathyroid hormone (PTH) levels ranged from 300 to 1,900 pg/ml with an average of 1,150 pg/ml (normal 50-340 pg/ml). At operation, a single parathyroid adenoma was discovered in 15 patients, and four-gland hyperplasia was noted in three patients. In all cases, serum levels of calcium returned to normal after operation. We conclude that patients with malignant tumors and concomitant hypercalcemia should be evaluated for the possibility of hyperparathyroidism. In cases of primary hyperparathyroidism, elevated C-terminal PTH level should be diagnostic. If hyperparathyroidism is determined to be the cause of hypercalcemia, neck exploration and parathyroidectomy are indicated.