Initial failure of surgical exploration in patients with primary hyperparathyroidism. To determine the causes of failures of cervical exploration for primary hyperparathyroidism, we reviewed 892 patients operated on by one surgeon from 1953 to 1990. Twenty-seven patients (3%) remained hypercalcemic or developed hypercalcemia within 6 months of surgery. Of these, five patients had one adenoma removed initially; at reoperation, three patients had a second adenoma that was successfully removed, whereas the other two patients had hyperplasia and required subtotal parathyroidectomies. No enlarged parathyroid glands were identified in 22 patients. Eventually, six patients became normocalcemic spontaneously, seven patients underwent re-exploration with a successful outcome in all but one case, two patients had ectopic hyperparathyroidism associated with carcinoma elsewhere, and seven patients refused reoperation and remain hypercalcemic. The failure rate of surgical exploration for primary hyperparathyroidism can be reduced by systematically exploring all four parathyroid glands. All abnormal parathyroids should be removed with histologic verification. When no abnormal glands are found, localization studies should be performed before re-exploration. 