The success of parathyroid surgery is determined by the identification and removal of all hyperactive parathyroid tissue. Ectopic location of parathyroid tumours and fibrosis due to previous operations can cause failure of parathyroidectomy. Parathyroid tumours accumulate and retain 2-methoxyisobutylisonitrile (MIBI) labelled with technetium-99m. This study assesses the value of intra-operative localization of parathyroid tumours using a hand-held gamma detector in patients with hyperparathyroidism and parathyroid cancer. Twenty patients undergoing their first operations for hyperparathyroidism, 15 patients undergoing reoperations for either persistent or recurrent hyperparathyroidism and two patients with parathyroid cancer were studied. Radioactivity in the neck and the mediastinum was recorded by a gamma detector after administration of 370 MBq 99m Tc-MIBI. Surgical findings and postoperative serum levels of calcium were documented. The sensitivity of the gamma detector in identifying parathyroid tumours was 90.5% in first parathyroidectomies, 88.9% in reoperations for either persistent or recurrent hyperparathyroidism and 100% in parathyroid cancer. One false-positive result was due to a thyroid nodule. Hypercalcaemia ceased in all but one patient postoperatively. It is concluded that employment of the gamma detector is to be advocated in first parathyroidectomies when a parathyroid tumour cannot be discovered, in reoperations for either persistent or recurrent hyperparathyroidism and in surgery for parathyroid cancer.