Forty five thallium-technetium parathyroid scans were performed preoperatively in patients with a high suspicion of parathyroid adenoma or hyperplasia. The patients were imaged after sequential IV injection of 201Tl-thallous chloride (74 MBq) and 99mTc-pertechnetate (222 MBq) using a gamma camera with a pinhole collimator. According to surgical findings, the prevalence of parathyroid abnormalities was 42/45: single (34 patients) and double (1 patient) adenomas, hyperplasia (7 patients, 16 hyperplastic glands). Three methods of analysis of scintigraphic images were compared. Method one was the visual comparison of the thallium and the technetium images. Method two used the subtraction image obtained after normalisation. Method three used a new image comparison method which performs the geometric and gray level registrations of the two images and generates the image of the significant differences. Three observers were involved in the evaluation procedures. Surgery and histological examinations were taken as the gold standard. Results show that the sensitivity is increased by applying method three. The interobserver reproducibility of the results is also higher for method three. We conclude that an adequate image subtraction technique significantly increases the diagnostic value of parathyroid scintigraphy.