Surgical exploration identified 40 patients with several enlarged parathyroid glands (two to five) out of a retrospective series of 500 cases of primary hyperthyroidism. Complete clinicopathological correlations were available in 27 cases (19 women and 8 men). Seven patients had a history of Wermer's syndrome (type I, MEN). Final diagnosis was as follows: double adenomas, 11 cases; primary hyperplasia, 16 cases. In two cases, there was a clear-cell hyperplasia, 16 cases. In two cases, there was a clear-cell hyperplasia with massive involvement of four parathyroid glands. The fourteen other cases presented with predominantly chief-cell hyper plasia; the involvement of the parathyroid glands was frequently asymmetrical, mainly or exclusively confined to two enlarged glands in 9 cases. The authors conclude that when a patient presents with two enlarged parathyroid glands, two diagnoses are possible--double adenoma or asymmetrical hyperplasia. Since anatomical study of the two enlarged glands is not always sufficient for accurate classification of the lesions, it is advisable to make a histological examination of a third.