The authors suggest a new method of biopsy of a seemingly normal human parathyroid gland during surgery for parathyroid adenoma. Such a method provides adequate surgical specimen to rule out a likely diffuse hyperplastic disease associated with the parathyroid adenoma, and for the ultrastructural study of the gland, without any risk of postoperative complication. In all patients, ultrastructural examination showed an abnormal cell population and signs of glandular atrophy as a consequence of the high level of serum calcium due to hormonal hyperfunction of the parathyroid adenoma.