[en] The merit of the preoperative localization of parathyroid tumors with thallium-technetium subtraction imaging is a subject of current debate in the treatment of primary hyperparathyroidism. Eighty patients with hyperparathyroidism underwent preoperative subtraction scintigraphy with ²⁰¹Tl Cl and /sup 99m/TcO₄; scan results were correlated with the operative identification and histopathology of the resected parathyroid tissue. The true-positive, false-positive and false-negative rates of these scans were compared between patients with tumors in normal and ectopic anatomic locations and between patients undergoing an initial and reoperative neck exploration. The scan was clearly valuable in patients with one or more prior neck explorations. True-positive scans were obtained in seven (77%) of nine such patients. The scan was also valuable in patients who had ectopic tumors (six mediastinal, seven intrathyroid, and two in the carotid sheath). Twelve (80%) of these 15 ectopic tumors were correctly localized either prior to their first operation or before subsequent explorations. In contrast, only 33 (50%) of 65 patients had a true-positive scan prior to their first operation and when the tumor was not in an ectopic location. In summary, in this series, the thallium-technetium scan was correct in only 50 per cent of patients undergoing an initial operation. However, it was positive in 77 per cent of patients who had at least one prior neck exploration and in 80 per cent of patients with an ectopic parathyroid tumor. These results support the selective use of this valuable imaging and localization tool