Mediastinal lymph nodes metastasis of lung cancer was analysed by endoscopic ultrasonography in 96 patients in whom histological diagnosis of the lymph nodes could be proven by thoracotomy. A Receiver Operating Characteristic (ROC) curve was drawn up for determining the criteria for metastatic lymph nodes by endoscopic ultrasonography. The ROC curve was evaluated by using four parameters: The long diameter, the short diameter, the long diameter plus the short diameter, and the product of the long diameter times the short diameter. The long plus short diameter and the long-times-short diameter showed the highest detectability of the metastasis of all histological types on the ROC curve. The reasonable criteria levels were found to be 18 mm and 75 mm2 by means of a moderate threshold. In this level, sensitivity were 77%, 76%; specificity 84%, 84%; and accuracy 82%, 82%, respectively. When examinations were done by histological types, the criteria level for the epidermoid carcinoma should be increased to more than that for the adenocarcinoma. And, sensitivity and specificity were better in the epidermoid carcinoma. Thus, these results suggest that endoscopic ultrasonography is clinically useful for the detection of lymph nodes metastasis in lung cancer.