Of the 178 consecutive mediastinoscopies performed at the Istituto Nazionale per lo Studio e la Cura dei Tumori of Milan from 1967 to 1974 145 were performed for diagnostic purposes and 33 for the pre-operative evaluation of resectability in cases of proven lung carcinoma. Mediastinoscopy is a very useful procedure for obtaining a definite pathological diagnsis in indeterminate mediastinal and pulmonary lesions. In succases, mediastinoscopy should precede any attempt at therapy, as 1/3 primary mediastinal lymph node lesions are benign and about 30% of suspected mediastinal tumors or lymphomas are actually misdiagnosed lung cancers (mainly oat-cell carcinomas). As to the role of mediastinoscopy in the assessment of lung cancer resectability, the current view is that it plays a part together with tomography and bronchoscopy in the clinical staging of this neoplasm, but it is no longer relied upon alone. It is mandatory in every case of suspected or probable mediastinal invasion and the criteria for inoperability are the following: spread to high paratracheal or contralateral nodes, direct invasion of the mediastinal fat, fixity of the neoplasm to the trachea or vertebral bodies. In such cases mediastinoscopy will prevent a useless exploratory thoracotomy.