Among patients with primary lung cancer who were admitted to the National Cancer Center Hospital from July 1987 to April 1988 for surgical treatments, 132 underwent preoperative transesophageal endoscopic ultrasound examination (EUS) on mediastinal lymph nodes. Of the 132 patients, 101 were pathologically evaluated and studied in this article. A GF-UM2 radial scanner with 7.5-MHz (Olympus Co Ltd) was used for image examination. The lymph nodes were diagnosed as positive for metastasis when they had thickened images, clear contours, and low echoing images of fusion or lobulation. The results obtained from 509 sites were as follows: sensitivity, 53.6 percent; specificity, 97.5 percent; positive predictive accuracy, 77.1 percent; negative predictive accuracy, 93.1 percent; and overall accuracy, 91.6 percent. The sensitivity rate was 80.6 percent excluding the result of the right superior mediastinal lymph nodes that were difficult to examine for anatomic reasons. Although EUS was considered to be an excellent method in diagnosing lymph node metastases, it had a blind angle in the field. More accurate diagnoses of mediastinal lymph node metastases could be achieved by using EUS and computed tomography (CT) together.