Six consecutive patients with ectopic parathyroid lesions were studied by preoperative thallium-technetium scintigraphy. Ectopic lesions were accurately localized in 5 of 6 patients. Four of five lesions deep in the superior mediastinum, and 1 in the superior carotid sheath, were correctly identified. Ectopic lesions in 4 of 5 patients, who had previously undergone unsuccessful exploration, were correctly localized. In 1 previously unoperated patient, a mediastinal lesion was identified by preoperative scintigraphy. In 2 patients, deep mediastinal exploration via manubriotomy was successfully conducted without confirmation by "invasive" intravascular procedures. There were no false-positive studies. Lesions identified weighed from 185 mg to 9 g and were from 1.1 to 3.0 cm in size. Scintigraphy is particularly effective for demonstration of ectopic parathyroid tissue, because the absence of thyroid tissue in the mediastinum or superior cervical region permits appreciation of areas of abnormal thallium uptake unobscured by uptake in adjacent thyroid tissue. We conclude that ectopic parathyroid tissue may be accurately identified by thallium-technetium scintigraphy without confirmation by invasive intravascular studies.