Abstract A case of relatively rare primary hyperparathyroidism due to a functioning adenoma of the parathyroid gland has been presented. Diagnosis of the disease was based upon (1) excision and biopsy of an osteolytic mandibular tumor which revealed hemosiderin and giant cells consistent with a “brown” tumor, or osteitis fibrosa cystica; (2) elevated serum calcium and depressed serum phosphorus; and (3) persistent and chronic renal lithiasis. The patient was subsequently operated upon and a large, solitary, functioning adenoma of the parathyroid was removed. The patient's serum calcium and phosphorus levels returned to normal limits and she has been asymptomatic since. A brief discussion of the endocrine physiology of the parathyroid has also been presented.