Lung cancer staging, based on anatomic extent of disease and described by the TNM staging system (T, primary tumor; N, regional lymph nodes; M, distant metastasis), is an important parameter for determining the clinical course of this disease. To evaluate the prognostic importance of TNM staging for lung cancer, we conducted a retrospective study analyzing survival rates according to TNM staging in 2,382 patients who had pulmonary resection for non-small cell lung cancer. Postoperatively, 3 patients were classified in stage 0, 796 in stage I, 304 in stage II, 719 in stage IIIA, 233 in stage IIIB, and 327 in stage IV. The 5-year survival rates for these patients were as follows: stage I, 68.5%; stage II, 46.9%; stage IIIA, 26.1%; stage IIIB, 9.0%; and stage IV, 11.2% (including ipsilateral, intrapulmonary metastases); 5-year survival rates for 140 patients with stage IV disease with intrapulmonary metastases in either the same lobe or another ipsilateral lobe were 17.8% and 8.3%, respectively. There was prognostic significance between stage I and stage II disease, stage II and stage IIIA disease, and stage IIIA and stage IIIB disease, but not between stage IIIB and stage IV disease. Only a few modifications will be required for the TNM staging system, which at present accurately reflects the prognosis of patients with lung cancer and is helpful in determining treatment.