Although p21 WAF1/Cip1 expression has been detected immunohistochemically in non-small cell lung cancer (NSCLC), the associations between p21 expression and clinical characteristics are unknown. To determine the association between p21 expression and clinical features, p21 expression was immunohistochemically analyzed in paraffin-embedded tumor samples from 137 patients with curatively resected NSCLC. p21 expression, indicating normal p21 function, was detected in 48 (35.0%) of the 137 patients with curatively resected NSCLC and was detected more frequently in patients with stage I or II disease (40.2%) than in those with stage IIIA disease (22.5%; P = 0.0483). There was no difference in the positive rate between squamous cell carcinoma [SCC; 15 of 48 (31.3%)] and adenocarcinoma [30 of 77 (39.0%)]. For SCC, patients with tumors expressing p21 survived longer than did those with tumors negative for p21 expression; however, the corresponding survival time was not significant for adenocarcinoma. On the other hand, p53 expression, detected in 58 (42.3%) of these patients, did not act as any predictor for prognosis in either SCC or adenocarcinoma. Our findings suggest that the presence of p21 expression is associated with favorable prognosis in SCC and may be useful in obtaining candidates for adjuvant therapies from among patients with SCC.