Randomized controlled trials have reported a 4–5 times increased risk of heart failure (HF) in breast cancer patients receiving trastuzumab (Herceptin®) compared to patients who do not receive trastuzumab. However, data regarding the cardiac effects of trastuzumab on elderly patients treated in general practice remain very limited. Using the US surveillance, epidemiology, and end results (SEER)-Medicare database, we conducted a retrospective cohort study on the cardiac effects of trastuzumab use in all incident breast cancer patients diagnosed from 1998 to 2007 who were 66 years and older, had no prior recent claims for cardiomyopathy (CM) or HF, and were followed through 2009. We defined our outcome as the first CM/HF event after diagnosis. We performed Cox-proportional hazard models with propensity score adjustment to estimate CM/HF risk associated with trastuzumab use. A total of 6,829 out of 68,536 breast cancer patients (median age: 75) had an incident CM/HF event. Patients who received trastuzumab tended to be younger, non-white, diagnosed more recently, and had a stage IV diagnosis. Trastuzumab use was associated with an increased risk of CM/HF (HR = 2.08, 95 % CI 1.77–2.44, p < 0.001). The trastuzumab-associated CM/HF risk was stronger in patients who were younger (HR = 2.52 for 66–75 years and HR = 1.44 for 76 years and older, p < 0.001) and diagnosed in recent years (HR = 2.58 for 2006–2007 vs. 1.86 for 1998–2005, p = 0.01). The twofold risk of CM/HF associated with trastuzumab remained regardless of patients’ diagnosis stage, presence of hypertension, cardiovascular comorbidities, or receipt of anthracyclines, taxanes, or radiation. Trastuzumab may double CM/HF risk among elderly breast cancer patients. Our findings reinforce the need to prevent and manage cardiac risk among elderly breast cancer patients receiving trastuzumab.