OBJECTIVES: The purpose of this study was to estimate incidence of and identify factors associated with cardiotoxicity, defined as heart failure and/or cardiomyopathy, and all-cause mortality in breast cancer patients undergoing adjuvant chemotherapy or hormones.  METHODS: A retrospective, population-based cohort study of 108,672 women (≥66 years of age) newly diagnosed with breast cancer from 2001-2009 was conducted using the Surveillance, Epidemiology, and End Results (SEER)-Medicare-linked database. Adjuvant chemotherapy were classified as mutually exclusive groups: trastuzumab-based, anthracycline-based, anthracycline and trastuzumab-based, taxane-based, and other chemotherapy. Propensity score matching adjusted for differences in patient characteristics across treatments. The final sample included a total of 11,250 women. Multivariable Cox proportional hazards regression models estimated hazard ratios (HRs) of cardiotoxicity and all-cause mortality with adjustment for inverse probability weights, sociodemographics, cancer characteristics, comorbidities, surgery and radiation, region, and year at diagnosis. RESULTS: Compared with hormones, risk of cardiotoxicity was higher in patients treated with anthracycline and trastuzumab-based (adjusted HR=1.87; 95% confidence intervals [CI]=1.51-2.33), trastuzumab-based (HR=1.32; 95%CI=1.14-1.52), and anthracycline-based (HR=1.14; 95%CI=1.03-1.27) regimens, respectively. Certain baseline characteristics were significant predictors of cardiotoxicity, including demographics (older age (vs. ≤70), non-hispanic black), cancer characteristics (advanced stage), comorbidities (cardiovascular conditions or renal failure), year at diagnosis, and West region (vs. Northeast). Additionally, risk of all-cause mortality was higher in patients treated with taxane-based (HR=1.54; 95%CI=1.43-1.67) regimens compared to hormones. Baseline characteristics including sociodemographics, cancer characteristics, cardiovascular or renal failure comorbid conditions, year at diagnosis, and South region were significant predictors of all-cause mortality (all P<0.05). CONCLUSIONS: Women with breast cancer treated with trastuzumab-based and/or anthracycline-based regimens had increased cardiotoxicity risk compared with hormones, while those treated with taxane-based regimens had higher rates of all-cause mortality. Types of chemotherapy are associated with increased risk of cardiotoxicity and all-cause mortality. Practitioners should further evaluate treatment and patient characteristics for risk mitigation strategies.