OBJECTIVE There are more than 2,000,000 breast cancer survivors in the United States today. While surveillance for asymptomatic recurrence and second primary is included in consensus recommendations, the effectiveness of this surveillance has not been well characterized. Our purpose is to estimate the effectiveness of surveillance mammography in a cohort of breast cancer survivors with complete ascertainment of surveillance mammograms and negligible losses to follow-up. METHODS We enrolled 1,846 stage I and II breast cancer patients who were at least 65 years old at six integrated health care delivery systems. We used medical record review and existing databases to ascertain patient, tumor, and therapy characteristics, as well as receipt of surveillance mammograms. We linked personal identifiers to the National Death Index to ascertain date and cause of death. We matched four controls to each breast cancer decedent to estimate the association between receipt of surveillance mammogram and breast cancer mortality. RESULTS One hundred seventy-eight women died of breast cancer during 5 years of follow-up. Each additional surveillance mammogram was associated with a 0.69-fold decrease in the odds of breast cancer mortality (95% CI, 0.52 to 0.92). The protective association was strongest among women with stage I disease, those who received mastectomy, and those in the oldest age group. CONCLUSIONS Given existing recommendations for post-therapy surveillance, trials to compare surveillance with no surveillance are unlikely. This large observational study provides support for the recommendations, suggesting that receipt of surveillance mammograms reduces the rate of breast cancer mortality in older patients diagnosed with early-stage disease.