Parathyroid tumors were first associated with the lesions of osteitis fibrosa cystica by Askanazy in 1904. Since that time more than 1,500 patient reports of hyperparathyroidism have appeared in the medical literature.1-3More than 80% of these cases are attributable to a single parathyroid adenoma. Another 5% to 10% are the result of two or more parathyroid adenomas identified at the initial neck exploration. Among the group with multiple parathyroid adenomas are included patients with primary chief cell hyperplasia. This is still a controversial entity which is difficult to distinguish microscopically from true adenoma, which may involve one or all of the parathyroid glands, and which has been frequently associated with multiple tumors of other endocrine organs. Instances of hyperplastic changes involving all the glands secondary to impairment of renal function or to deficient calcium absorption from the gastrointestinal tract account for most of the remaining cases. Parathyroid adenocarcinoma