Authors analysed and present the results of CT in evaluating preoperative staging, of non-Small cell lungconcer, especially focusing mediastinal lymph node metastasis and direct invasion to the mediastinum or chestwall. All 28 cases were thoroughly assessed by open thoracotomy and mediastinoscopy in Seoul Natinal UniversityHospital during recent 3 years. The results are as follows: 1. 26 cases were male and 2 cases were female withpeak age incidence of 6th decade. 2. Histopathological type were 20 cases of squamous cell carcinoma, 6 cases ofadenocarcinoma, 1 case of undifferentiated large cell carcinoma and 1 case of adenosquamous cell carcioma. 3.Overall prevalence rate of mediastinal lymph node metastasis was 37%. Prospective metastatic lymph node evaluationusing 15mm size criterior revealed sensitivity 56%, specificity 92%, accuracy 76%. Retrospective analysis using10mm size criterior revealed sensitivity 90%, specificity 53%, accuracy 67%, while the results of 15mm sizecirterior were sensitivity 60%, specificity 88%, accuracy 78%. 4. Plotted ROC curve with 109 mediastinal lymphnodes suggested optimum size criterior of metastasis being between 10mm and 15mm in diameter. 5. Prevalence rateof direct mediastinal or chest wall invasion was 31%. Results of CT interpretation concerning direct mediastinalor chest wall invasion revealed sensitivity 63%, specificity 100%, accuracy 88%. Authors believe from the basis ofthese results that CT palys unique and reliable role in preoperative staging of non-small cell carcinoma of lungand emphasize CT should be a routine preoperative staging work up.