PURPOSE: A cohort study was performed to investigate the carcinogenic effect of treatment of skin hemangioma with ionizing radiation in early childhood. This paper presents the incidence of breast cancer (BC) in this cohort and its association with radiotherapy. METHODS AND MATERIALS: In an incidence study, 3,316 women treated for a skin hemangioma between 1941 and 1977 at the Institut Gustave-Roussy were included, among whom 2,697 had received radiotherapy. The mean age at first exposure was 0.7 years, and the mean absorbed dose to the breast was 70 mGy. Treatment reconstruction and the estimation of radiation doses delivered to the breast were obtained for 92% of the women who had received radiotherapy. External and internal analyses were performed. RESULTS: During an average follow-up of 35 years, a total of 17 women developed an invasive BC, compared to 7.5 expected in the French general population (SIR = 2.3, 95% CI, 1.4-3.5), and the absolute excess risk strongly increased with attained age. Compared to individuals with no radiotherapy, the risk of BC increased with increasing radiation dose with RRs of 3.2, 6.3, and 8.0 for dose categories of >0-10, 10-100, and >100 mGy, respectively; however, dose-response relationship was not significant. CONCLUSION: This study confirms that radiation treatment performed in the past for hemangioma during childhood increases the risk of BC.