Purpose Clinical trials indicate that postmastectomy radiation therapy (PMRT) improves survival for women age younger than 70 years with high-risk breast cancer. However, for women age 70 years or older, the benefits of PMRT are unknown. As recent evidence suggests that certain adjuvant treatments appropriate for younger women may only be marginally beneficial for older women, we sought to determine whether PMRT improves survival for older women with breast cancer. Methods Using the Surveillance, Epidemiology and End Results Medicare data spanning 1992 to 1999, we identified 11,594 women age 70 years or older treated with mastectomy for invasive breast cancer. A proportional hazards model adjusted for clinical-pathologic covariates tested whether PMRT was associated with improved overall survival for low-risk (T1/2 N0), intermediate-risk (T1/2 N1), and high-risk (T3/4 and/or N2/3) patients. Results A total of 502 (7%) of 7,416 low-risk, 242 (11%) of 2,145 intermediate-risk, and 785 (38%) of 2,053 high-risk patients received PMRT. Median follow-up was 6.2 years. For low- and intermediate-risk patients, PMRT was not associated with survival. For high-risk patients, PMRT was associated with a significant improvement in survival (hazard ratio, 0.85; 95% CI, 0.75 to 0.97; P = .02). Five-year adjusted survival was 50% for patients not treated with PMRT or chemotherapy, 56% for patients treated with PMRT only, 57% for patients treated with chemotherapy only, and 59% for patients treated with both PMRT and chemotherapy. Conclusion PMRT is associated with improved survival for older women with high-risk breast cancer. Randomized clinical trials are urgently needed to confirm this finding and define optimal treatment strategies for this patient group.