As risk for secondary breast cancer is elevated among cancer survivors treated with conventional therapy, we sought to determine the risk among 3337 female 5-year survivors who underwent an allogeneic hematopoietic cell transplantation (HCT) at the Fred Hutchinson Cancer Research Center or at one of 82 centers reporting to the European Bone Marrow Transplant Registry. Risk was calculated using standardized incidence ratios (SIRs), and risk factors were evaluated with a multivariable Cox proportional hazards model. Fifty-two survivors developed breast cancer at a median of 12.5 (range: 5.7-24.8) years following HCT (SIR=2.2). Twenty-five-year cumulative incidence was 11.0%, higher among survivors who received total body irradiation (TBI) (17%) than those who did not receive TBI (3%). In multivariable analysis, increased risk was associated with longer time since transplantation (hazard ratio [HR] for 20+ years after transplantation=10.8), use of TBI (HR=4.0), and younger age at transplantation (HR=9.5 for HCT<18 years). Hazard for death associated with breast cancer was 2.5 (95% CI: 1.1-5.8). We conclude that female survivors of allogeneic HCT are at increased risk of breast cancer and should be educated about the need for regular screening.