Radiotherapy following breast-conserving surgery for the treatment of first primary breast cancer is the standard of care and is widely used despite its small survival benefit. The effects of radiotherapy in metachronous contralateral breast cancer are unknown. We examined the use of radiotherapy and its effect on cause-specific and all-cause mortality among women with metachronous contralateral breast cancer treated with breast-conserving surgery in community settings. Using data from the 1985–2000 Surveillance, Epidemiology, and End Results program, we identified women with stage 0-III metachronous contralateral breast cancer that occurred at least six months after stage 0-III first primary breast cancer. Cause-specific and all-cause mortality of women age 40–69 who did and who did not receive radiotherapy following breast-conserving surgery for metachronous contralateral breast cancer were compared in proportional hazard models using propensity scores to balance covariates by radiotherapy use. We adjusted for misclassification of radiotherapy use. Based on misclassification-corrected analyses, 43.2 percent of 1,083 women with metachronous contralateral breast cancer did not receive radiotherapy after BCS. After adjustment for propensity scores and radiotherapy misclassification, women who did not receive radiotherapy had 2.2 times greater risk of cause-specific and 1.7 times greater risk of all-cause mortality. In community settings, a high percentage of women with stage 0-III metachronous contralateral breast cancer did not receive radiotherapy following breast-conserving surgery. Unlike the small survival benefit of radiotherapy after first primary breast cancer, omission of radiotherapy after metachronous contralateral breast cancer significantly increased the risk of cause-specific and all-cause mortality.