Background The incidence and management of trastuzumab-mediated cardiotoxicity outside of clinical trials has not been well described. Objective and methods The aim of the study was to retrospectively evaluate the incidence of cardiac dysfunction, characterize its natural history, and identify the degree of reversibility using cardiac MRI, in a population of HER-2 positive breast cancer patients receiving trastuzumab in the adjuvant setting. Results Out of 152 patients (mean age 52 +/- 10 years), 36 (24%) developed trastuzumab mediated cardiomyopathy, the majority asymptomatic. Factors that predicted the development of trastuzumab mediated cardiac dysfunction were a pre-existing history of hypertension, smoking history, and a family history of coronary artery disease. Within 3 months of treatment with trastuzumab, there was a difference in LVEF between the normal cohort and those patients who developed LV systolic dysfunction (61 +/- 5% vs. 51 +/- 8%, P < 0.01). During the 6-month-followup, 34/36 patients demonstrated subepicardial linear delayed enhancement of the lateral wall of the left ventricle on cardiac MRI, suggesting trastuzumab induced myocarditis. Conclusion Approximately 1 in 4 women may develop LV systolic dysfunction after treatment with adjuvant trastuzumab, necessitating careful patient selection and close serial monitoring using noninvasive cardiac imaging.