Of 53 patients who had hyperparathyroidism assocated with multiple parathyroid gland enlargement, 39 (74%) had primary hyperparathyroidism without clinical or laboratory evidence of associated endocrine gland dysfunction, 2 had documented familial primary hyperparathyroidism, and 12 had hyperparathyroidism as part of the multiple endocrine neoplasia syndrome. When last studied, 31 of the 39 patients with nonfamilial hyperparathyroidism had normal serum calcium levels, 3 had permanent hypoparathyroidism, 2 had recurrent hyperparathyroidism, and 3 were lost to follow-up. The two patients with familial hyperparathyroidism were treated by removal only of enlarged parathyroid glands, and in each, hyperparathyroidism recurred. Five patients with multiple endocrine neoplasia, type 1, were treated by removal only of enlarged parathyroid glands, and hyperparathyroidism recurred in four. Four patients with multiple endocrine neoplasia, type 1, were treated by removal of three or more parathyroid glands, and there were no instances of recurrent hyperparathyroidism. In one patient, permanent hypoparathyroidism developed. Three patients with multiple endocrine neoplasia syndrome, type 2, had total parathyroidectomies as a part of thyroidectomy for medullary thyroid carcinoma. In each patient, permanent hypoparathyroidism developed. When primary hyperparathyroidism occurs in the absence of a definite history of polyendocrine or familial disease, only the glands that are definitely enlarged should be removed, and normal-appearing glands should be tagged rather than risk the possibility of permanent hypoparathyroidism that may attend routine subtotal parathyroid gland excision.