PURPOSE: To investigate breast cancer risk after supradiaphragmatic radiotherapy administered to young women with Hodgkin's lymphoma (HL) in a much larger cohort than previously to provide data for patient follow-up and screening individualized according to treatment type, age, and time point during follow-up. PATIENTS AND METHODS: Breast cancer risk was assessed in 5,002 women in England and Wales treated for HL with supradiaphragmatic radiotherapy at age < 36 years from 1956 to 2003, who underwent follow-up with 97% completeness until December 31, 2008. RESULTS: Breast cancer or ductal carcinoma in situ developed in 373 patients, with a standardized incidence ratio (SIR) of 5.0 (95% CI, 4.5 to 5.5). SIRs were greatest for those treated at age 14 years (47.2; 95% CI, 28.0 to 79.8) and continued to remain high for at least 40 years. The maximum absolute excess risk was at attained ages 50 to 59 years. Alkylating chemotherapy or pelvic radiotherapy diminished the risk, but only for women treated at age ≥ 20 years, not for those treated when younger. Cumulative risks were tabulated in detail; for 40-year follow-up, the risk for patients receiving ≥ 40 Gy mantle radiotherapy at young ages was 48%. CONCLUSION: This article provides individualized risk estimates based on large numbers for patients with HL undergoing follow-up after radiotherapy at young ages. Follow-up of such women needs to continue for 40 years or longer and may require more-intensive screening regimens than those in national general population programs. Special consideration is needed of potential measures to reduce breast cancer risk for girls treated with supradiaphragmatic radiotherapy at pubertal ages.