The aim of our retrospective study is to evaluate the value of Computed Tomography (CT) of the chest, for the preoperative staging of the mediastinum, in Non Small Cell Lung Cancer (NSCLC) patients, when mediastinal and hilar lymph nodes of more than 1 cm in diameter are not apparent in CT. During the three years study period (1996-1998), one hundred fifty patients underwent thoracotomy and pulmonary parenchyma resection for Τ1-2Ν0Μ0 primary lung cancer, according to bronchoscopic findings and chest, brain, abdomen CT and bone scanning findings. Formal mediastinal lymph node dissection was performed in all 150 patients. Twenty seven patients (18%), with preoperative staging Τ1-2Ν0Μ0, had metastatic involvement to ipsilateral mediastinal lymph nodes (N2 disease) at microscopic examination. Out of these patients, 14,7% had squamous cell carcinoma, 20,83% adenocarcinoma and 20% large cell carcinoma. Formal mediastinal lymph node dissection modifies preoperative lung cancer staging and prognosis in serious percentage (18%) of NSCLC patients who are preoperatively classified as Τ1-2Ν0Μ0 and consequently impose the need for postoperative adjuvant therapy. Poorly differentiated neoplasms, independent of cell type, have increased chance of mediastinal lymph node involvement, despite the normal size of lymph nodes in chest CT. Pneumon 2000, 13 (3): 213-218