Endosonography in the localization of parathyroid tumors: a preliminary study. A preoperative transesophageal exploration of the parathyroids by endosonography was performed on 23 patients with primary hyperparathyroidism. The system used was a 7.5 MHz transducer mounted on the tip of an endoscope with an external diameter of 13 mm. The field of visualization was 360 degrees. A retrograde exploration was done moving up from the aortic arch to the upper esophageal sphincter. All patients underwent surgery afterward, and adenomas were found. In 12 cases the adenoma was visualized. All 12 adenomas were posteriorly located on the right side (four cases) and left side (eight cases) of the esophagus. Nine of these 12 tumors were on the posterior face of the thyroid lobes, with six tumors in the middle one third of the thyroid lobe and three in the lower one third of the thyroid lobe. The other three tumors were located below the lower pole of the thyroid lobes in the upper posterior mediastinum. Mean tumor weight was 1165 mg. Of the 11 tumors that could not be visualized, eight tumors were anteriorly located; three of these tumors were on the anterior and lateral surface of the lower pole of the thyroid, and five were in the thyrothymic tracts. The remaining three tumors were located on the back of the thyroid lobes; two of these tumors were at the upper esophageal sphincter, and one was on the side of the pharynx. Mean tumor size was 1334 mg. Localization of parathyroid tumors by endosonography appears possible but only if lesions are located posteriorly, close to the esophagus. Endosonography is not indicated before routine cervical exploration for primary or secondary hyperparathyroidism. As in other such studies, endosonography could be useful in cases of persistent or recurrent hyperparathyroidism. 