A patient with sarcoidosis and chronic renal failure was treated for hyperphosphatemia with aluminum hydroxide. The subsequent fall in serum phosphorus was followed by the development of hypercalcemia and nephrolithiasis. Corticosteroid therapy normalized the serum calcium and halted the progression of the nephrolithiasis, but did not improve renal function. Hyperphosphatemia may have blocked the expression of sarcoid hypercalcemia in the patient. The mechanism is unclear but inhibition of the synthesis or action of 1,25-dihydroxyvitamin D may have been involved. Reduction of serum phosphorus may lead to severe hypercalcemia in some patients with sarcoidosis.