Six cases of primary hyperparathyroidism due to hyperfunctioning intrathyroidal parathyroid glands are reported. In five cases, hyperparathyroidism was due to an intrathyroidal parathyroid adenoma; in the sixth case, hyperparathyroidism resulted from an intrathyroidal parathyroid carcinoma. All five patients with adenoma were female with ages ranging from 40 to 70 yr. The patient with carcinoma was a 55-yr-old male. In all five patients with intrathyroidal parathyroid adenoma, thyroidectomy was performed when an abnormal parathyroid gland could not be located in the neck during surgery for hyperparathyroidism. The patient with intrathyroidal parathyroid carcinoma presented with hypercalcemia and a palpable right thyroid mass. The differential diagnosis of intrathyroidal parathyroid adenoma includes thyroid follicular adenoma. In some cases, the possibility of medullary carcinoma of thyroid might also be considered. Immunocytochemical staining for parathormone (PTH), thyroglobulin, and calcitonin is valuable in establishing the correct diagnosis.