Background: Trastuzumab-based therapy (TT) is standard treatment for HER2-positive breast cancer (HBC). Subclinical cardiotoxicity (SCTx), defined as asymptomatic decline in left ventricular ejection fraction (LVEF) > 10% to < 50%, has been reported in up to 30 % of HBC patients (pts) receiving TT. Objectives included: determine prevalence of SCTx; associated risk factors (RF); and completion rates of TT in pts with HBC referred to a cardio-oncology clinic (COC). Methods: HBC patients receiving TT referred to the Ottawa Hospital COC were included. Demographics, TNM staging, performance status, stage, cardio-vascular (CV) RF (history of heart disease, hypertension, smoking, dyslipidemia, and diabetes), cardiac medications (CM) (ACE-inhibitors, beta-blockers), baseline LVEF, previous cancer therapy, baseline anthracycline exposure, previous radiation therapy (RT) (including mediastinal RT) were collected. LVEF was evaluated by ECHO or MUGA. Rate of successful completion of TT among pts with SCTx was determined. Risk ratio (RR) and logistic regression analysis was performed. Results: 240/408 BC pts referred to the COC (2008-2016) had HBC and 163/240 (68%) were referred with SCTx while on TT. 139/163 (85 %) pts with SCTx recovered after COC assessment: 77/163 (47%) pts were prescribed CMs. A significantly higher proportion of recovery was observed in pts who did not require CM (0.92 vs 0.78, p = 0.012; RR = 0.85, 95%CI:0.74-0.91). A total of 129/163 (79%) pts who experienced SCTx finished a full course of TT. Regression analysis found baseline LVEF, diabetes, and diastolic blood pressure as significant RFs for SCTx. There were no independent predictors for recovery after asymptomatic drop in LVEF while on TT. Diabetes (OR:2.97, 95%CI:1.3-6.8) and left chest wall RT (OR:2.4, 95%CI:1.1-5.6) significantly increased risk of permanent TT interruption in pts with asymptomatic drop in LVEF. Conclusions: The majority of HBC pts who experience SCTx can safely complete a full course of TT; many without use of CMs. While CV RFs were associated with increased risk of SCTx, this did not impact CV recovery after asymptomatic drops in LVEF.