In this study, the extent to which CT accuracy in the diagnosis of regional lymph node metastasis of non-small cell lung cancer contributed to treatment results with radiation therapy was examined. The long-termed survival rate of 133 cases examined by CT scanning was superior to that of 154 cases diagnosed by conventional radiological method alone. The difference was statistically significant in N2 cases. When the mediastinal lymph node metastasis was detected on the ipsilateral side of the primary tumor or when the mediastinal lymph node swelling was smaller than 3 cm in diameter, the prognosis became better.