Radiotherapy (RT) is the standard therapy for unresected stage I-II nonsmall cell lung cancer (NSCLC). Using population-based data, we compared survival and toxicity among unresected elderly patients treated with combined chemoradiotherapy (CRT) or RT alone. Using the Surveillance, Epidemiology and End Results (SEER) registry (National Cancer Institute, Bethesda, MD, USA) we identified 3,006 cases of unresected stage I-II NSCLC. We used propensity score methods to compare survival and rates of toxicity of patients treated with RT versus CRT. Overall, 844 (28%) patients received CRT. Adjusted analyses showed that CRT was associated with improved survival (hazard ratio 0.85, 95% CI 0.78-0.94). Combination therapy was also associated with better survival among stage I patients treated with intermediate complexity RT (HR 0.80, 95% CI 0.70-0.90); however, no difference in survival was observed among patients treated with complex RT. In stage II patients, CRT was associated with improved survival regardless of the RT technique (HR 0.61-0.72). CRT was associated with increased odds of toxicity. Despite increased toxicity, CRT may improve survival of elderly unresected patients with stage II disease as well as stage I NSCLC treated with intermediate RT complexity. Randomised trials are needed to clarify the balance of benefits and risk of CRT in unresected patients.