Technetium-99m-sestamibi (MIBI) is a new radionuclide for imaging parathyroid tissue. The purpose of this study was to evaluate parathyroid localization using single radiotracer, dual-phase MIBI scintigraphy and to compare the results to ultrasonography. Twenty-one patients with hyperparathyroidism underwent dual-phase scintigraphy using 25 mCi MIBI and high resolution ultrasonography before parathyroidectomy. Scan results were correlated with size, weight, location, and histopathology of excised parathyroid glands, thyroid abnormalities, and cost. Seventeen patients were female, five had secondary or tertiary hyperparathyroidism, and three had a previous parathyroid exploration. Twenty patients (95%) were cured, 14 with a single and 1 with a double adenoma, and 5 of 6 patients with generalized hyperplasia. There were no false positive MIBI scans and one false positive ultrasound study, despite associated thyroid nodules in 29 per cent of patients. The sensitivity of MIBI and ultrasound in the identification of adenomas was 87 per cent versus 57 per cent (P = 0.046), and the rate of detection of hyperplastic glands was 44 per cent versus 24 per cent (P = 0.19), respectively. There was no correlation between scan results and size, weight, or location of adenomatous glands. The cost of dual-phase MIBI was comparable to that of ultrasound. Dual-phase MIBI is more sensitive than ultrasound in the localization of adenomas and is the preferable modality for preoperative parathyroid localization. Neither MIBI nor ultrasound is effective in localization of hyperplastic glands, underscoring the importance of routine bilateral neck exploration.