The mediastinum was studied by endoscopic ultrasonography in 121 lung cancer patients. This method facilitates the observation of the lymph nodes, the large vessels and heart in the mediastinum in real time and dynamically. In this paper, the orientation of mediastinal lymph nodes was ultrasonographically studied. This method can detect lymph nodes as small as 3 mm in diameter, and very highly those in bracheobronchial, subaortic, subcarinal, and hilar regions. The mediastinal vascular structures were also easily detected. This method can provide full clinical applicability for analysis of mediastinal involvements in lung cancer patients.