Background The major limitation in the use of trastuzumab therapy is cardiotoxicity. We evaluated the safety of a strategy of continuing trastuzumab in patients with breast cancer despite mild, asymptomatic left ventricular impairment.Methods Charts of consecutive patients referred to a cardio-oncology clinic from January 2015 to March 2017 for decline in left ventricular ejection fraction (lvef), defined as a fall of 10 percentage points or more, or a value of less than 50% during trastuzumab therapy, were reviewed. The primary outcome of interest was change in lvef, measured before and during trastuzumab exposure and up to 3 times after initiation of cardiac medications during a median of 9 months.Results All 18 patients referred for decline in lvef chose to remain on trastuzumab and were included. All patients were treated with angiotensin converting–enzyme inhibitors or beta-blockers, or both. After initiation of cardiac medications, lvef increased over time by 4.6 percentage points (95% confidence interval: 1.9 percentage points to 7.4 percentage points), approaching baseline values. Of the 18 patients, 17 (94%) were asymptomatic at all future visits. No deaths occurred in the group.Conclusions Many patients with mildly reduced lvef and minimal heart failure symptoms might be able to continue trastuzumab without further decline in lvef, adverse cardiac events, or death when treated under the supervision of a cardiologist with close follow-up.