A unique patient with simultaneous idiopathic hypoparathyroidism and active Paget's disease is described. This patient demonstrated that endogenous parathyroid hormone (PTH) is not essential for bone resorption in this disease. Treatment with exogenous PTH resulted in restoration of serum calcium and phosphorus to normal and enhanced bone resorption as evidenced by increased urinary hydroxyproline and calcium excretion. A similar response to exogenous PTH was observed in a euparathyroid subject with Paget's disease in whom severe transient hypercalcemia was also produced. Blockade of PTH action by mithramycin therapy in the euparathyroid subject lowered urinary hydroxyproline excretion precipitously but also resulted in elevation of serum PTH which contributed to a secondary increase of urinary hydroxyproline. It was concluded that Paget's disease can be maintained despite deficiency of PTH but that activation of the Pagetic process can occur in the presence of increased PTH. The therapeutic implications of this responsiveness of Pagetic bone to PTH are discussed.