There is now widespread evidence that female survivors of Hodgkin's disease who have been treated with supradiaphragmatic radiotherapy are at an increased risk of breast cancer. Mantle irradiation, which includes irradiation of the mediastinum, conveys a particularly high risk. Previously published studies have found a wide variation in risk. To provide British estimates of risk to inform surveillance programmes, we carried out the first British population-based cohort study of breast cancer in female survivors of childhood Hodgkin's disease. From the underlying cohort of the British Childhood Cancer Survivor Study, a cohort of 18,123 British 5-year survivors of childhood cancer diagnosed between 1940 and 1991, there were in total 383 female 5-year survivors of childhood Hodgkin's disease. Sixteen of these 383 survivors went on to develop invasive breast cancer subsequent to 5-year survival (standardised incidence ratio, 11.5; 95% confidence interval (95% CI), 6.6-18.6) and all of these 16 survivors had been treated with supradiaphragmatic irradiation as treatment for childhood Hodgkin's disease. The cumulative risk of breast cancer by 25 years of follow up was 9.9% (95% CI, 3.3-16.6) for all patients and 12.2% (95% CI, 4.3-20.1) for those treated with supradiaphragmatic radiotherapy. The cumulative risk of breast cancer in female survivors of childhood Hodgkin's disease in Britain is at the lower end of previous estimates. We hope that our data may provide a basis for future surveillance and for counselling survivors as to their likely risk of breast cancer. (c) 2006 Wiley-Liss, Inc.