A prospective, controlled, randomized trial on the value of mediastinoscopy was performed during a 12-month period (1969-1970) on 130 patients with a bronchogenic carcinoma. Patients to be admitted to the study had to meet the criteria of being clinically operable. 126 patients could be followed up for the following 5 years. Of these patients 62 had a mediastinoscopy previous to the thoracotomy (Group A) and 64 had directly a thoracotomy (Group B). A curable resection was possible in Group A in 56.4% and in 73.5% in Group B. The perioperative mortality (30 days) is 8% for Group A and 16% for Group B. The mean survival rate is for patients with a positive mediastinoscopy which did not underwent thoracotomy with 6.8 months better than for patients which underwent directly an exploratory thoracotomy (2.7 months). The 5-year survival rate is despite the high perioperative mortality for Group B (16%)higher than for Group A(10%). Mediastinoscopy is with its low mortality and its better short surviving rate of great value for high risk patients and inoperable patients with bronchogenic carcinomas, but reduces the overall resectability and the 5-year survival rate.