PURPOSE Female survivors of childhood cancer have a high risk of subsequent breast cancer. We describe the ensuing risk for mortality and additional breast cancers. PATIENTS AND METHODS Female participants in the Childhood Cancer Survivor Study, a cohort of 5-year survivors of cancer diagnosed between 1970 and 1986 before age 21 years, and subsequently diagnosed with breast cancer (n = 274; median age at breast cancer diagnosis, 38 years; range, 20 to 58 years) were matched to a control group (n = 1,095) with de novo breast cancer. Hazard ratios (HRs) and 95% CIs were estimated from cause-specific proportional hazards models. RESULTS Ninety-two childhood cancer survivors died, 49 as a result of breast cancer. Overall survival after breast cancer was 73% by 10 years. Subsequent risk of death as a result of any cause was higher among childhood cancer survivors than among controls (HR, 2.2; 95% CI, 1.7 to 3.0) and remained elevated after adjusting for breast cancer treatment (HR, 2.4; 95% CI, 1.7 to 3.2). Although breast cancer–specific mortality was modestly elevated among childhood cancer survivors (HR, 1.3; 95% CI, 0.9 to 2.0), survivors were five times more likely to die as a result of other health-related causes, including other subsequent malignant neoplasms and cardiovascular or pulmonary disease (HR, 5.5; 95% CI, 3.4 to 9.0). The cumulative incidence of a second asynchronous breast cancer also was elevated significantly compared with controls ( P < .001). CONCLUSION Mortality after breast cancer was higher in childhood cancer survivors than in women with de novo breast cancer. This increased mortality reflects the burden of comorbidity and highlights the need for risk-reducing interventions.