In presenting the staging system, I recognize that in a given patient the total tumor burden cannot be precisely quantitated, and the balance between host defenses and the heterogeneity of the malignancy is not measurable. These and other complex interacting biological variables will influence the subsequent course of the disease. However, our data support the premise that the straightforward indices of disease extent in the TNM system permit a simple yet valid classification that best reflects prognosis. Patients can be grouped together according to certain measurable common features of their disease so that within each stage group treatment options and survival expectations will be generally similar. In this manner reliable and valid comparisons of the results of different modalities of therapy can be made. Survival data according to staging criteria are a measure of the efficacy of available therapy for lung cancer; thus, the staging information serves as a valuable guide for treatment planning.