Survival outcome of 1592 analyzable patients on four Radiation Therapy Oncology Group (RTOG) studies in inoperable non-small cell lung cancer were studied utilizing a recursive partitioning analysis (RPA). This approach creates a regression tree according to prognostic variables which partitions into homogenous subsets according to survival. Four protocols, RTOG 83-11, 83-21, 84-03 and 84-07 were analyzed. 83-11 and 84-07 were studies utilizing irradiation with alterfractionation; 83-21 and 84-03 were studies evaluating thymocin with irradiation and prophylactic cranial irradiation with thoracic irradiation respectively. Nine pretreatment variables and one treatment variable were analyzed. Adjustment for radiotherapy effect was made in the accelerated treatment protocol (84-07). Overall, median survival for the entire group was 9.0 months with 17% alive at 2 years. Univariate analysis suggests that KPS, < or = 70 vs. 80-100, pleural effusion, weight loss, < or = 5% vs. 5%, age, 60+ vs. < 60, T stage (T1 and T2 vs. T3 and T4) and N stage (N- vs. N+) were important prognastic factors. Radiation dose, sex, race and histology were not univariate prognastic factors. RPA identified KPS as the most significant covariate (median survival 5.9 mos. < or = 70 vs. 9.9 mos. 80-100). Within KPS 80-100 other splits occurred for N stage, age, weight loss and radiation therapy dose. KPS < or = 70 split at pleural effusion only. The best overall RPA tree has four distinct classes with median survival times ranging from 3.3 to 12.6 months. The RPA classes were validated in an independent non-small cell lung cancer dataset. This analysis may allow more intelligent stratification and study-design for future RTOG trials in inoperable non-small cell lung cancer.