Introduction: Although the risk factors for lung cancer are well documented, whether previous radiation therapy (RT) to treat breast cancer (BC) increases the incidence of second primary lung cancer remains unclear. We determined the overall incidence of second primary lung cancer after previous RT for all clinical stages of BC. Methods: After an analysis of the Longitudinal Health Insurance Database, 986,713 individuals were randomly enrolled from 2000 to 2010 for cohort analysis. All women with newly diagnosed invasive BC, aged 18 years or older, and without other types of malignancy before 2000 or in the follow-up periods were enrolled and allocated to the RT or non-RT groups. Age, comorbidities, insurance premium, urbanization level, location, date of event, and hospital care level were analyzed. The hazard ratios of parameters were calculated by Cox regression analysis. Results: A total of 7408 individuals met the inclusion criteria, including 5695 patients with BC who underwent RT and 128 (2.25%) in whom lung cancer developed. In contrast, lung cancer was diagnosed in only four of 1713 individuals in the non-RT group (0.23%) during the followup period. Cox regression analysis showed that the adjusted hazard ratio was 10.078 times higher in the RT group than in the non-RT group for individuals with previous BC. Conclusions: The incidence of second primary lung cancer was significantly higher in the RT group than in the non-RT group for individuals with previous BC. Patients with BC treated by RT should be extensively surveyed to assess the incidence of subsequent lung cancer. (C) 2017 International Association for the Study of Lung Cancer. Published by Elsevier Inc. All rights reserved.