OBJECTIVE In Japan, with the initiation of the lung cancer screening program, small-sized peripheral lung cancer in which the diameter is 2 cm or less has been increasing. The purpose of this study is to determine the clinicopathologic behavior of small-sized lung cancer. METHODS Four hundred ninety-six patients with cT1 N0, peripheral, resected non-small-cell lung cancer, who were operated on between 1980 and 1996, were selected, grouped by tumor diameter or histologic type, and then analyzed for clinicopathologic behavior. On the basis of measured diameter roentgenographically, the patients were divided into two groups; group c-S with lesions 2 cm or less in diameter and group c-L with lesions 2.1 to 3 cm in diameter. RESULTS Lymph node metastasis was recognized in 18% of group c-S, in 23% of group c-L, and in 21% for the entire clinical group. The rate of those with the progressive state was 19% in group c-S and 26% in group c-L. The 5-year survival was 79.5% in group c-S and 69.3% in group c-L (i.e., there was a significant difference between the two groups). CONCLUSIONS Compared with the patients with lesions 2.1 to 3 cm in diameter, the patients with small-sized lung cancer had a milder progressive state and a better prognosis.