It is well known that when parathyroid extract, the active principle of the parathyroid gland, is injected into an organism, hypercalcemia is produced. If these injections are continued a negative calcium balance results, with ultimate decalcification. This is followed by the formation of fibrous bone lesions, as Jaffe, Bodansky and Blair1have recently shown. That a condition similar to this experimental hyperparathyroidism could exist clinically was not clearly recognized until Mandl2in 1926 removed a parathyroid tumor in a case of generalized osteitis fibrosa cystica and thereby effected a prompt and apparently permanent clinical improvement. This suggested that the causative factor of osteitis fibrosa cystica was a hyperactive parathyroid. The correctness of this conclusion has been fairly well established by a series of other cases reviewed by Barr and Bulger,3and correlated under the term "clinical hyperparathyroidism." In this paper we are presenting another case of osteitis