Background: Human epidermal growth factor receptor-2 (HER2) is overexpressed in 20e25% of breast cancers. Complete eradication of disease following neoadjuvant therapies and chemotherapy has been referred to as pathological complete response (pCR). Aims: To determine clinicopathological predictors of pCR to neoadjuvant therapies and to evaluate pCR as a surrogate to enhanced survival. Methods: Consecutive female patients with HER2 positive (HERþ) breast cancer managed surgically in a single institution between 2005 and 2015 were included. Descriptive statistics and binary logistic regression were used to determine predictors of pCR. Appraisal of pCR as a predictor of survival was performed using Kaplan-Meier curves and Cox regression analysis. Results: 451 patients were included with a mean age of 56.6 ± 13.4 years (range 23e95). Disease-free (DFS) and overall survival (OS) was 82.3% (371/451) and 82.6% (376/451) respectively with a median follow-up of 108.0 months (range 3e184.0). 118 were treated in the neoadjuvant setting (26.2%): tumour size <50 mm (Odds Ratio (OR): 12.156, P ¼ 0.023) and progesterone receptor negativity (OR: 2.762, P ¼ 0.008) independently predicted breast pCR, while ductal carcinoma (OR: 3.203, P ¼ 0.030) and grade 3 disease (OR: 2.788, P ¼ 0.018) predicted axillary pCR. Both breast and axillary pCR predicted enhanced DFS (Hazard Ratio (HR): 0.470 & HR: 0.449) and OS (HR: 0.383 & HR: 0.307). Axillary pCR independently predicted improved OS (HR: 0.326). Conclusion: pCR is sensitive biomarker and surrogate to survival outcomes in HER2þ breast cancer. Patients likely to achieve pCR may be predicted from traditional clinicopathological characteristics and molecular parameters.

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