Treatment outcome of 63 patients younger than 50 years of age initiated on a course of once-daily definitive radiation therapy without concurrent or preirradiation chemotherapy for clinical Stages I-III unresected non-small cell lung carcinoma from 1978 to 1988 was compared to the outcome of 695 patients over the age of 50. Follow-up ranged from 24-110 months with follow-up until death in 88% of patients. The actuarial overall survival rate for all patients was 22% at 2 years with a median survival time of 11.5 months. Patients less than 50 and greater than or equal to 50 years old were similar in male:female ratio, distribution of histologic subtype, performance status, and extent of weight loss. Poorly differentiated histologic grade was more prevalent among the younger patients (59% vs 41%, p = .005). Ninety-four percent of younger patients and 86% of older patients had clinical stage III disease (p = NS). Survival was significantly worse for patients who were younger than 50 years old (p = .05), with a median survival time of 7.8 months. Median survival time for those patients 50 years of age or older was 12.4 months. Poorer survival outcome among young patients was most pronounced among patients with unfavorable characteristics of poor performance status (greater than or equal to 2) or weight loss (greater than 5%) (p = .002). Distant failure (p = .029) and brain failure (p = .003) as initial site of relapse was more common among younger patients. Among young patients, poor histologic grade was associated with both distant failure (p = .003) and brain metastasis (p = .002). The difference in distribution of histologic grade, incidence of distant failure, particularly in the brain, and poorer survival outcome among patients less than 50 may be indicative of more aggressive tumor behavior in the younger patients. These results indicate that patients less than 50 may require alternate treatment strategies. Age should be considered a stratification variable in non-operative randomized trials of non-small cell lung carcinoma which include patients with non-favorable characteristics.