Tumors with a maximum dimension of 3 cm are categorized as T1, whereas those greater than 3 cm are T2 by TNM classification. Some physicians suggest that early-stage peripheral lung cancer should have a maximum tumor diameter of 2 cm and that limited surgery (segmentectomy without lymph node dissection) is acceptable for the patients. In this study, the relationship between the tumor dimension and prognosis was analyzed in 207 patients with surgically treated primary non-small-cell lung cancer (SCLC). The 5-year survival rate of those with tumors 3 cm or less and without lymph node (LN) metastases was 86%, which was significantly higher than that of those with tumors more than 3 cm and without hilar and mediastinal LN metastases (65%) (p < 0.05). However, 33% of the patients with tumors 3 cm or less had LN metastases, and the 5-year survival rate did not differ between those with tumors 3 cm or less (60%) and those with tumors more than 3 cm (54%). Twenty-eight percent of patients with tumors 2 cm or less had LN metastases, and the 5-year survival rate of the patients with tumors 2 cm or less was 62%. The 5-year survival rate of those with tumors 2 cm or less and without LN metastases was 88%. Forty-six patients with tumors 2 cm or less included 5 cases with an intrapulmonary metastasis in the same lobe (11%). In conclusion, a size of 3 cm is an appropriate boundary as the T factor. Because those with tumors 2 cm or less have a relatively high percentage of LN metastases, intraoperative frozen sections of LN should be considered for those undergoing limited surgery for primary non-SCLCs 2 cm or less. Intrapulmonary metastases also should be considered for those undergoing limited surgery even if the maximum dimension of the primary tumor is less than 2 cm.