Forty-seven (39%) of 120 patients with presumably operable lung cancer were found to have metastatic tumor in mediastinal lymph nodes by mediastinoscopy. Unnecessary thoracotomy was avoided in these 47 patients. Results of anterior mediastinal node biopsy were positive in 16 (38%) of 42 patients with primary tumors of the left lung who were evaluated by anterior mediastinoscopy without resection or division of costal cartilage. Thirty-one (40%) of 78 patients with primary tumors of the right lung had positive results of mediastinal node biopsy as demonstrated by cervical mediastinoscopy. Sixty-seven percent of patients with centrally located tumors, 43% with adenocarcinoma, and 57% with undifferentiated carcinoma had mediastinal lymph node metastasis. Negative results of anterior mediastinoscopy in patients with primary tumors of the left lung predicted resectability in 25 (96%) of 26 instances. Forty-three (91%) of 47 patients with primary tumors of the right lung and negative results of cervical mediastinoscopy had resectable tumors. Anterior mediastinoscopy appears to be as accurate in the evaluation of mediastinal lymph node metastasis in left lung cancer as cervical mediastinoscopy is in the evaluation of right lung cancer.