Of 121 patients with primary hyperparathyroidism, 20% demonstrated hyperfunction in more than one parathyroid gland. Patterns of gross enlargement and microscopic hyperplasia varied greatly in patients with multiple parathyroid gland involvement. Persistence of hypercalcemia (2.6% this series) due to failure to remove all abnormal parathyroids is a greater problem than late recurrence (less than 1% this series), which is usually due to a single hyperplastic hyperparathyroid. To minimize persistent or recurrent hypercalcemia as well as permanent postoperative hypoparathyroidism (4% this series), a policy of selective subtotal parathyroidectomy is advocated. Subtotal parathyroidectomy is performed when multiple parathyroids are grossly enlarged or when all parathyroids are slightly enlarged and a fifth parathyroid is not evident, and is planned for conditions predisposing to multiple gland involvement (multiple endocrine adenoma syndrome, familial hyperparathyroidism, mild chronic renal insufficiency).