BACKGROUND: Chemotherapy with trastuzumab and anthracycline is associated with incident heart failure (HF) in patients with breast cancer. We hypothesized that continuous incidental use of β-blocker agents (BB) was protective against HF in patients without established structural heart disease who were receiving trastuzumab and anthracycline. METHODS AND RESULTS: We identified 920 consecutive patients with breast cancer (age 52.3±11.0 years) with normal ejection fraction before receiving trastuzumab and anthracycline therapy at our institution between 2005 and 2010. Using a propensity score and a greedy 5 to 1 digit-matching algorithm, 106 of these patients on continuous BB during cancer treatment were matched with 212 patients from the same pool with similar characteristics but not on continuous BB. During a median follow-up of 3.2±2.0 years, 32 incident HF admissions were identified in these 318 patients with breast cancer, whereas 28 cancer-related (noncardiac) deaths occurred before any incident HF. Cumulative incidence regression models and cause-specific hazards of new HF events were estimated from competing risk Cox models of time-dependent covariates. Although trastuzumab therapy showed significant association with incident HF, independent of anthracycline-related cardiotoxicity (hazard ratio, 9.0; 95% confidence interval, 3.0-27.0; P<0.0001), continuous use of BB was associated with lower risk of new HF events (hazard ratio, 0.2; 95% confidence interval, 0.1-0.5; P=0.003). CONCLUSIONS: Coincidental, continuous use of BB is associated with lower incidence of HF in patients with breast cancer and normal baseline ejection fraction in a competing risk framework, and after matching for demographics, clinical, and cancer-related treatment characteristics. Prospective randomized clinical trials to validate these findings are warranted.