The staging and histologic cell type of patients in the Lung Cancer Study Group (LCSG) clinical trials program are reviewed and confirmed or resolved at the reference center for anatomic and pathologic classification of lung cancer. A high level of consistency in classification has been achieved through the use of criteria that minimize intraobserver variability. The data obtained from the review project have been used to characterize the relationship of disease extent and cell type to survival in the clinical trials population. Survival characteristics were generated for 1,121 patients who underwent apparent complete resection of nonsmall cell lung cancer and were subsequently entered into various protocols to receive either adjuvant treatment or no further therapy. The end results study provides some insight regarding the biological behavior of squamous cell carcinoma and adenocarcinoma of the lung in terms of the anatomic extent of disease at the time of apparent complete resection. Patients with squamous cell carcinoma had an outcome superior to that of patients with adenocarcinoma in every TNM subset. The differences in survival according to these major cell types were significant overall and in the T1 N0, T1 N1, and T2 N1 subsets but not in the TNM subsets in stage III disease. Histologic cell type and extent of disease are important factors in survival expectations; thus the accuracy and reproducibility of these classifications plays a significant role in the evaluation of differing modalities of treatment.