The entity of preclinical hyperparathyroidism has never been clearly investigated. The authors believe that the incidence of pathologic abnormalities of the parathyroid glands before the development of any symptoms or hypercalcemia (serum calcium > 12.0 mg/dl) is more frequent than has been reported. Over a 14-year period, parathyroid glands were examined during thyroid operations in over 800 patients. Serum calcium and phosphorous levels were measured in all patients preoperatively. Thirty-six patients had additional parathyroid operations for a preclinical form of hyperparathyroidism, defined by abnormal appearing parathyroid glands at the time of thyroid surgery. None of the 36 patients had symptoms of hyperparathyroidism preoperatively. Nine patients had borderline hypercalcemia (serum calcium 10.6 to 12.0 mg/dl), and the remainder were considered normocalcemic. The average age was 53 (range 21 to 75) with a male to female ratio of 1:3. Nine of the 36 patients had thyroid cancer. There were eight patients with parathyroid adenoma and 28 patients with parathyroid hyperplasia. Of 13 patients who had a history of neck irradiation, five had parathyroid adenoma and eight had parathyroid hyperplasia. Only two patients with parathyroid hyperplasia remain on calcium medication. Since preoperative normocalcemia does not preclude the presence of parathyroid pathology, the authors urge careful identification and examination of the parathyroid glands during thyroid operations. It adds little time to the procedure. Excision of parathyroid disease along with the thyroid gland can be performed safely and prevents the need for further operation with its associated morbidity.