Intractable hypercalcemia is the major cause of morbidity and mortality in patients with parathyroid carcinoma. Because gallium nitrate previously was shown to inhibit the bone resorptive activity of parathyroid hormone (PTH) in vitro, we used it to treat two patients with parathyroid carcinoma and resistant hypercalcemia. In both patients, total serum calcium levels were reduced from initial values of 3.62 and 3.77 mmol/L to posttreatment values of 2.32 and 1.45 mmol/L, respectively. Urinary excretion of calcium and hydroxyproline also declined significantly. Serum PTH levels were lower in both patients after therapy, although all levels remained markedly elevated. Nephrogenous cyclic adenosine monophosphate and tubular reabsorption of phosphate remained unchanged. These data indicate that treatment with gallium nitrate can control hypercalcemia in patients with high circulating levels of PTH. Gallium nitrate antagonizes the bone resorptive activity of PTH without altering renal effects of the hormone.