Eighteen patients with hyperparathyroidism underwent high-resolution computerized tomography of the neck and superior mediastinum to assess the accuracy of the method in localizing diseased parathyroid glands preoperatively. The tomograms were correlated with surgical and pathological findings. Four scans were technically unsatisfactory. Of the remaining 14 scans, 8 showed an enlarged parathyroid gland; in 7 cases, the scan correctly identified the location of the diseased gland while in 1 case, the side of the lesion was incorrect. The other six scans did not show an abnormal parathyroid. These glands varied in size from 0.05 to 5.0 cm3 at the time of pathological examination. Three patients were undergoing re-exploration for persistent disease; technically acceptable scans were obtained in two and, in both, the location of the adenoma was correctly identified. Excluding patients with technically unsatisfactory scans, this technique has a sensitivity of 50% (7/14) and a false-negative rate of 43% (6/14) independent of gland size. However, in the eight scans interpreted as positive, the correct side of the lesion was localized in 7 (88%). This technique is not recommended for routine preoperative localization but may be of value in re-exploration for persistent disease.