['To test the hypothesis that radiotherapy (RT) improves the outcome of patients with unresected, nonmetastatic bronchoalveolar carcinoma (BAC) by performing a population-based analysis within the Surveillance, Epidemiology, and End Results (SEER) registry.', 'Inclusion criteria were as follows: patients diagnosed with BAC, Stage I-III, between 2001 and 2007. Exclusion criteria included unknown stage, unknown primary treatment modality, Stage IV disease, and those diagnosed at autopsy. Demographic data, treatment details, and overall survival were retrieved from the SEER database. Survival was analyzed using the Kaplan-Meier method and log-rank test.', 'A total of 6933 patients with Stage I-III BAC were included in the analysis. The median age at diagnosis was 70 years (range, 10-101 years). The majority of patients were diagnosed with Stage I (74.4%); 968 patients (14%) did not undergo surgical resection. Unresected patients were more likely to be older (p < 0.0001), male (p = 0.001), black (p\xa0< 0.0001), and Stage III (p < 0.0001). Within the cohort of unresected patients, 300\xa0(31%) were treated with RT. The estimated 2-year overall survival for patients with unresected, nonmetastatic BAC was 58%, 44%, and 27% in Stage I, II, and III, respectively. Factors associated with improved survival included female sex, earlier stage at diagnosis, and use of RT. Median survival in those not receiving RT vs. receiving RT was as follows: Stage I, 28 months vs. 33\xa0months (n = 364, p = 0.06); Stage II, 18 months vs. not reached (n = 31, nonsignificant); Stage III, 10 months vs. 17 months (n = 517, p < 0.003).'] Does radiotherapy improve survival in unresected stage I-III bronchoalveolar carcinoma?