Of 116 mediastinoscopies performed over a period of 5 years without serious complications, 36 were for diagnosis of obvious or suspected mediastinal and hilar lymphadenopathy. In 34 of these 36 procedures large samples of paratracheal lymph nodes were excised for histological and partially also for bacteriological examination. In 24 patients a definite diagnosis was obtained (mediastinoscopy positive). There were 17 cases of sarcoidosis among them. The investigation was negative in 10 other patients. In 6 of these the negative result was fully explained by the final diagnosis (true negative mediastinoscopy). Only in 4 of the 34 patients a different disease affecting the lymph system was discovered by further investigations (false negative). All four patients had neoplastic disease. In accordance with the results of other investigators, the author's findings show that suspicion of sarcoidosis can be confirmed mediastinoscopically in almost 100% of cases. However, mediastinoscopy was not equally successful in diagnosing Hodgkin's disease and malignant lymphoma. The paratracheal lymph nodes were affected only in approximately half of these patients, although other intrathoracic lymph nodes, e.g. the anterior and/or posterior nodes, were shown to be involved. In these cases diagnosis can be obtained only by thoracotomy and/or staging laparotomy.