BACKGROUND Several case reports have suggested an association between radiation therapy for breast cancer and the subsequent occurrence of esophageal carcinomas. OBJECTIVE To examine the association between radiation therapy for breast cancer and subsequent esophageal squamous-cell carcinomas and adenocarcinomas. DESIGN Population-based retrospective cohort study. SETTING Population-based U.S. cancer registries. PATIENTS 220,806 women in whom breast cancer was diagnosed between 1 January 1973 and 31 December 1993; 1,216,853 person-years of follow-up. MEASUREMENTS Age- and period-adjusted standardized incidence ratio as a measure of relative risk for the second primary cancer. RESULTS In women who had received radiation therapy for breast cancer, the relative risk for esophageal squamous-cell carcinoma increased to 5.42 (95% CI, 2.33 to 10.68) and the relative risk for esophageal adenocarcinoma increased to 4.22 (CI, 0.47 to 15.25) 10 or more years after radiation therapy. No increased risk was seen for either type of carcinoma among patients with breast cancer who did not receive radiation therapy. CONCLUSION The risk for esophageal carcinoma is increased in women who receive radiation therapy for breast cancer.