Misinterpretation of the upper parathyroid adenoma on thallium-201/technetium-99m subtraction scintigraphy. Pre-operative thallium-technetium subtraction scintigraphy of 50 patients with primary hyperparathyroidism, which was subsequently proven on histology to be due to an adenoma, was compared with operative findings. Six scans were negative and seven uninterpretable. Of the remaining 37 patients, 34 had correct scan prediction of the anatomical locations of the adenomas. On examination of the origins of the adenomas, all six scintigraphic predictions of upper adenomas corresponded to tumours arising from the upper gland found at operation. Of the 25 lower predictions, there was one false positive and 16 were correct, i.e. of lower gland origin, but eight were found to originate from the upper gland. This misinterpretation is though to be due to the prolapse of an adenoma arising from the upper gland towards the lower pole of the thyroid, thereby mimicking a lower lesion on the scan. 