In 36 patients with biochemical and clinical evidence of primary or secondary hyperparathyroidism (HPT), preoperative scintigraphic studies were performed with a thallium-technetium subtraction technique. The patients were given 30 MBq technetium pertechnetate and, after a delay of 10-15 minutes, 55 MBq thallium chloride. Data were collected with a gamma camera equipped with a pinhole collimator and dedicated computer. Images were recorded simultaneously in two channels, in order to provide identical positioning for the thallium and technetium images. A standardized gradual computer subtraction was then carried out. Parathyroid adenoma was present in 28 patients, primary parathyroid hyperplasia in two, and secondary hyperplasia due to chronic renal failure in six. The scintigrams located 24 (86%) of the adenomas, but only four (13%) of the total 32 hyperplastic glands. The scintigraphic technique offers considerable advantages in the preoperative location of parathyroid adenomas, which may be of particular interest in persistent or recurrent HPT.