PURPOSE To examine the association between total body irradiation (TBI) and subsequent breast cancer in women treated with blood or marrow transplantation (BMT) for hematologic malignancies. PATIENTS AND METHODS Participants were drawn from the BMT Survivor Study (BMTSS), a retrospective cohort study that included patients who underwent transplantation between 1974 and 2014 and survived for $ 2 years after BMT. Patients with pre-BMT chest radiation or a history of breast cancer were excluded. Participants completed the BMTSS survey, which included details regarding breast cancer diagnosis. Subsequent breast cancer was confirmed by pathology report review or physician notes. Cox proportional hazards models assessed the association between TBI and subsequent breast cancer. Standardized incidence ratios were calculated to determine the excess risk of subsequent breast cancer compared with that in the general population. RESULTS A total of 1,464 female BMT survivors (allogeneic: n 5 788; autologous: n 5 676) participated, with a median follow-up of 9.3 years from BMT. TBI was used in 660 patients (46%). Thirty-seven women developed subsequent breast cancer (allogeneic: n 5 19; autologous: n 5 18). Multivariable analysis revealed that exposure to TBI was associated with an increased risk of subsequent breast cancer among allogeneic BMT survivors (hazard ratio [HR], 3.7 [95% CI, 1.2 to 11.8]; P 5 .03) and autologous BMT survivors (HR, 2.6 [95% CI, 1.0 to 6.8]; P 5 .048). Pre-BMT exposure to alkylating agents was associated with an increased risk of subsequent breast cancer among autologous BMT survivors (HR, 3.3 [95% CI, 1.0 to 9.0]; P 5 .05). Compared with that in the general population, exposure to TBI at age, 30 years was associated with a 4.4-fold higher risk of subsequent breast cancer in allogeneic BMT survivors and a 4.6-fold higher risk in autologous BMT survivors. CONCLUSION The association between TBI and subsequent breast cancer, especially among those exposed at a young age, as well as pre-BMT exposure to alkylating agents, should inform breast cancer screening for early detection.