['To determine whether technetium Tc 99m sestamibi scanning is accurate enough to allow surgeons to perform unilateral neck exploration for first-time parathyroidectomy in patients with primary hyperparathyroidism.', 'Retrospective review.', 'University tertiary care center.', 'Forty patients with primary hyperparathyroidism who underwent sestamibi scanning before first-time parathyroidectomy, of whom 28 had single adenomas, 9 had multiple adenomas, and 3 had hyperplasia.', 'All 40 patients underwent bilateral neck exploration with identification of 4 parathyroid glands.', 'We compared the results of preoperative sestamibi scanning with operative and histologic findings. We then used these data to calculate the projected success rates of parathyroidectomy if unilateral neck explorations had been performed based on the results of sestamibi scanning, instead of bilateral explorations.', 'Sestamibi scanning was correct in 20 (71%) of 28 patients with single adenomas, 4 (44%) of 9 patients with multiple adenomas, and 0 (0%) of 3 patients with hyperplasia. If unilateral neck explorations had been performed on the basis of localization by sestamibi scanning, parathyroidectomy would have failed in 4 (10%) of 40 patients.'] Is sestamibi scanning inadequate for directing unilateral neck exploration for first-time parathyroidectomy?