Background: The purpose of this study is to determine a more refined T definition for lung cancer staging on the basis of clinical outcomes. Methods: The Walter Reed Army Medical Center Tumor Registry and the Thoracic Surgery Tumor Clinic files were queried for lung cancers diagnosed from 1990 to 2000. Cox regression analysis and Kaplan-Meier survival curves for tumor size were used to analyze the impact of size on survival and relative risk, and then used to redefine T. Using the new T definition, the cohort was restaged, and the two staging system survivals were compared using Cox regression analysis. Results: Tumor size was documented in 439 males and 226 females. Forty-two tumors were 1.0 cm or less, 133 were between 1.01 and 2.0 cm, 133 were between 2.01 and 3.0 cm, 91 were between 3.01 and 4.0, 96 were between 4.01 and 5.0, and 166 were greater than 5.0 cm. A survival advantage was noted for smaller lesions, with 5-year survivals of 48.6%, 45.9%, 26.6%, 27.0%, 14.4%, and 11.6%, respectively. Cox regression analysis revealed increased risk at 2.0 cm (hazards ratio, 2.014; 95% confidence interval, 1.24 to 3.26), 4.0 cm (hazards ratio, 2.51; 95% confidence interval, 1.53 to 4.09), and 5.0 cm (hazards ratio, 3.14; 95% confidence interval, 1.96 to 5.02). After redefining T, the new staging system showed a better 5-year survival in each stage. Conclusions: Lung cancer tumor size criteria should be changed to include T1 tumors 2.0 cm and less; T2 tumors between 2.0 and 4.0 cm or pleural invasion of T1 tumor; T3 tumors greater than 4.0 cm or pleural invasion of T2 tumors.