To determine the role of mediastinoscopy in the preoperative staging of the primary lung cancer, we studied 23 patients from January 1993 to December 1993 and compared the results of mediastinoscopy and computed tomographic scanning with the findings at thoracotomy. Mediastinoscopy was performed in 14 patients when they had larger than 10mm mediastinal nodes at computed tomographic scanning. Six of them were found to have metastatic nodes. Nine patients, who had negative computed tomographic scanning and negative mediastinoscopic results,underwent thoracotomy. One of them was found to have a metastatic mediastinal lymph node. When results from both series of patients were compared, the sensitivity and specificity of computed tomographic scanning showed 83.3% and 47.1% respectively with an accuracy of 56.5%. The results of mediastinoscopy showed that the sensitivity was 85.7%, the specificity was 100% and the accuracy was 95.7%.Because of the low accuracy rate of computed tomographic scanning, a more routine use of mediastinoscopy seems to be justified.