For patients with previous thyroid or parathyroid surgery and for those with significant comorbid conditions, noninvasive and invasive modalities exist for the preoperative localization of pathologic parathyroid tissue. Formal localization at our institution involves obtaining two studies that are independently positive for the same location. The studies utilized have included ultrasound, CT scans, MRI, technetium-99m/thallium-201 (Tc-Tl) imaging, and more recently, (99m)Tc-sestamibi (20-25 mCi) (MIBI) scans. These were followed by arteriography and/or venous sampling if necessary. From January 1992 through October 1995, 25 patients underwent preoperative parathyroid localization (10 reoperation, 3 grave hypercalcemia, 2 concurrent goiter, 2 cerebral vascular accident, 1 bleeding disorder, and 1 malignant ventricular arrhythmia) and were evaluated prospectively during the changeover from Tc-Tl to MIBI scanning at our institution. A total of 92 studies were obtained. All 25 patients were operated on by a single surgeon, and in each case the parathyroid adenoma was successfully resected with minimal morbidity (1 permanent hypoparathyroid and 1 temporary recurrent laryngeal nerve injury). True positives interpreted preoperatively: MIBI, 14 of 19 (74%); CT scan, 13 of 19 (68%); Tc-Tl, 4 of 8 (50%), ultrasound, 9 of 20 (45%); MRI, 8 of 14 (57%); arteriography, 3 of 7 (43%); venous sampling, 3 of 4 (75%); and positron emission tomography, 0 of 1. There were no false-positive MIBI scans. These results suggest that when formal parathyroid localization is needed in reoperative/complicated patients, 1) MIBI appears to be the most sensitive and specific study, and 2) the MIBI scan should be the initial study in any situation in which preoperative localization is needed. Ultrasound can be useful for patients with significant comorbidities who have not been previously explored. CT or MRI should then be used if further evaluation is needed. If the use of these noninvasive modalities does not produce positive gland localization, selective venous sampling should be utilized as the definitive procedure.