A child with a pheochromocytoma had hypercalcemia but no evidence for excessive parathyroid hormone secretion from the parathyroid glands or the pheochromocytoma. Therapy with the catecholamine synthesis inhibitor metyrosine (alpha-methyltyrosine) reversed the catecholamine excess but had no effect on the hypercalcemia. Adrenalectomy promptly reversed the hypercalcemia. Extracts of the tumor contained a substance(s) that produced both potent in-vitro bone resorption and striking adenylate-cyclase-stimulating activity in renal cortical membranes. This stimulating activity was due to activation of the parathyroid hormone receptor/adenylate cyclase complex but was not due to parathyroid hormone. Our findings document hypercalcemia in association with pheochromocytoma and show that hypercalcemia occurred in the absence of previously proposed mechanisms. We also provide preliminary characterization of the presumed responsible substance(s) and suggest that this substance(s) may be related to that associated with the humoral hypercalcemia of malignancy.