To assess the ability of ultrasonography (US) and dual tracer thallium-technetium subtraction scanning (NS) to localize abnormal parathyroid glands, these two investigations were carried out preoperatively in 27 consecutive patients who underwent surgery for hyperparathyroidism. Nineteen patients had adenomas and 8 hyperplasia. Of 23 US procedures 2 were inadequate, and of 26 NS procedures 1 was inadequate. Ultrasonography was found to be superior to NS for preoperative localization of abnormal parathyroid glands (sensitivity per gland 53% versus 36%); detection rates for hyperplasia were poor for both techniques (sensitivity per gland 36% and 25%). However, when positive, both techniques were extremely accurate (positive predictive value of 100% for both). There was no correlation between the weight of the gland or degree of physiological hyperfunction (parathyroid hormone level) and detection rates for the two techniques. False-positive results were rare for both, so a positive result was highly predictive of an abnormality at that location. Ultrasonography had sufficient accuracy to suggest its routine use when adenoma is suspected, particularly to detect the side of the lesion (sensitivity 78% and positive predictive value 100%). The addition of subtraction scintigraphy does not appear to be warranted.