Authors analysed and present the results of CT in evaluating preoperative staging, of non-Small cell lung concer, especially focusing mediastinal lymph node metastasis and direct invasion to the mediastinum or chest wall. All 28 cases were throughly assessed by open thoracotomy and mediastinoscopy in Seoul National University Hospital during recent 3 years. The results are as follows: 1. 26 cases were male and 2 cases were female with peak age incidence of 6th decade. 2. Histopathological types were 20 cases of squamous cell carcinoma, 6 cases of adenocaroinoma, 1 case of undifferentiated large cell carcinoma and 1 case of adenosquamous cell carcinoma. 3. Overall prevalence rate of mediastinal lymph node metastasis was 37%. Prospective metastatic lymph node evaluation using 15mm size criterior revealed sensitivity 56%, specificity 92%, accuracy 76%. Retrospective analysis using 10mm size criterior revealed sensitivity 90%, specificity 53%, accuracy 67%, while the results of 15 m size criterior were sensitivity 60%, specificity 88%, accuracy 78%. 4. Plotted ROC curve with 109 mediastinal lymph nodes suggested optimum size criterior of metastasis being between 10mm and 15mm in diameter. 5. Prevalence rate of direct mediastinal or chest wall invasion was 31%. Results of CT interpretation concering direct mediastinal or chest wall invasion revealed sensitivity 63%, specificity 100%, accuracy 88%. Authors believe from the basis of these results that CT plays unique and reliable role in preoperative staging of non-small cell carcinoma of lung and emphasize CT should be a routine preperative staging work up.