Between 1979 and 1984, mediastinoscopy was performed on 1,000 of the 1,500 patients admitted to the Thoracic Surgical Service of the Toronto General Hospital with the diagnosis of carcinoma of the lung. In 144 cases, concomitant anterior mediastinoscopy was also performed. Abnormal mediastinal nodes were found in 296 (29.6%). The overall complication rate was 2.3%, with no deaths. Mediastinoscopy revealed diseased nodes in 24% of patients with squamous cell carcinoma, 29% with adenocarcinoma, 54% with small cell undifferentiated carcinoma, 31% with large cell undifferentiated carcinoma, and 12% with bronchoalveolar carcinoma. Abnormal mediastinal nodes were found with equal frequency in right- and left-sided tumors and occurred in 31% of tumors in the main bronchus, 25% of upper lobe tumors, and 17% of lower lobe tumors. Of the 704 patients having negative results of mediastinoscopy, 590 were subjected to thoracotomy. Ninety-three percent underwent resection (85% curative, 7% palliative) and 7% had unresectable tumors. Of the resections, 20% were pneumonectomies. At thoracotomy, 52 of the 590 patients with negative mediastinoscopic results were found to have abnormal mediastinal nodes. Sixty-two of the 296 patients with positive results of mediastinoscopy were selected for thoracotomy. Eighty-six percent had resectable lesions (67% curative, 18% palliative) and 14% unresectable. The pneumonectomy rate in this group was 35%. These current data support our previous opinion that routine mediastinoscopy can be done with negligible morbidity and provides essential information for the classification and management of cancer of the lung.