In an autopsy study of 503 cases the parathyroid glands were dissected, and the number of glands in each case and the anatomic distribution of the glands were recorded. In 18 cases (3%) only three glands were found. In these cases the lower combined weight suggested that a fourth gland had been missed. In 421 cases (84%) there were four glands and in 64 cases (13%) there were supernumerary glands. Most often the supernumerary gland was a fifth gland, usually in the thymus. The anatomic distribution of the glands showed considerable constancy. The positions of the glands on the one side were symmetrical with those on the other side in approximately 80% of cases. The superior parathyroids were frequently found just above the intersection between the recurrent laryngeal nerve and the inferior thyroid artery. The inferior parathyroids most often lay somewhat more ventrally, close to the lower thyroid pole or in the upper thymus or thyrothymic ligament. In a few cases the lower parathyroids were situated higher up in the neck, obviously because of a failure of descent during the embryologic development. In view of the number of supernumerary glands and their location, it is concluded that wide excision of fat tissue surrounding the parathyroids and thymectomy should be performed during operation in patients with hyperparathyroidism secondary to uremia or those with multiple endocrine neoplasia syndromes.