Between 1976 and 1984, 242 patients with presumably operable lung cancer were treated surgically. In the Canisius Wilhelmina Hospital, Nijmegen, The Netherlands, in the period 1976 to 1980, 109 of 131 (83.2%) patients underwent cervical mediastinoscopy to assess operability. They were studied retrospectively. During this examination, lymph node metastasis was demonstrated in three of 19 (15.8%) patients with left upper lobe lung cancer. At thoracotomy after a normal cervical mediastinoscopic study or no mediastinoscopic study, periaortic lymph node metastases were found in eight of 34 (23.5%) patients with left upper lobe lung cancer. In the period 1981 to 1984, the value of left parasternal mediastinoscopy was studied prospectively in patients with left lung cancer in the Canisius Wilhelmina Hospital, Nijmegen; in the Lung Centre of the Radboud University Hospital, Nijmegen; and in the Lung Center of the Dekkerswald Medical Centre, Groesbeek. Cervical or cervical and parasternal mediastinoscopy were performed in 69 of 111 (62.2%) patients. At parasternal mediastinoscopy performed after a normal cervical mediastinoscopic study, periaortic lymph node metastases were found in seven of 31 (22.6%) patients with left upper lobe lung cancer. All periaortic lymph node metastases showed intranodal and extranodal growth. The resectability rate in left upper lobe lung cancer was 79.4% in the retrospective group and 96.5% in the prospective group. There were no serious complications after parasternal mediastinoscopy. These data point to the reliability of parasternal mediastinoscopy in the assessment of left upper lobe lung cancer. The study provides essential information for the staging and treatment of non-small cell lung cancer of the left upper lobe.