Because of the frequent topographically unusual localization of parathyroid tumours the intraoperative identification remains a problem to the surgeon. Therefore, the accuracy of the preoperative localization of the parathyroid glands was assessed in 17 patients with extrarenal hyperparathyroidism using high resolution sonography (Siemens RA 1 and Sonoline 8000 small parts scanner operating at 7.5 mHz/5 MHz). The clinically suspected diagnosis was supported by changes in calcium, phosphate and alkaline phosphatase levels without any signs of renal dysfunction and by elevated levels of PTH in the 44-68 hPTH radioimmunoassay. All patients subsequently underwent surgery and the preoperative sonographic diagnosis could be confirmed in 82% of the cases. The smallest adenoma visualized measured 0.8 X 0.8 cms. The only carcinoma in the study did not appear sonographically different to the adenomata. These preliminary results suggest that high resolution sonography may become a valuable aid in the surgical management of extrarenal hyperparathyroidism.