Double-tracer scanning (Tc-99m as pertechnetate and Tl-201 as chloride) with an Anger camera and computerized image subtraction was performed in 61 patients whose clinical and biochemical findings had suggested primary hyperparathyroidism. This study showed intra- or extrathyroidal focal uptake of thallium in 37 cases. Among these, 24 patients underwent surgery, and 18 parathyroid adenomas, five carcinomas, and one hyperplastic gland were found exactly in the sites predicted by scintigraphy. Among 24 patients with negative scans, only two underwent surgery; a hyperplastic parathyroid gland (diam less than 0.5 cm) was found in both. The success rate was 92% in the cases in which operation was performed. We believe that the new method may be useful in the preoperative detection of parathyroid enlargements.