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Background: The addition of trastuzumab (T) and lapatinib (L) to neoadjuvant chemotherapy increases the pathological complete response (pCR) rate in patients with human epidermal growth factor receptor 2 (HER2)-positive early breast cancer. We investigated the efficacy of T or L with neoadjuvant chemotherapy and specific efficacy biomarkers. Methods: Patients with stages I-III (including inflammatory) HER2-positive breast cancer were randomised to receive epirubicin (E) plus cyclophosphamide (C) x 4 cycles followed by docetaxel (D) plus either T (EC-DT) or L (EC-DL). End points included pCR (primary), clinical response, toxicity, and pCR-predictive biomarkers. Results: We randomised 102 patients to EC-DT (50) and EC-DL (52). Median age was 48, 56% were premenopausal and 58% had oestrogen receptor (ER)-positive tumours. Pathological complete response in breast was 52.1% (95% CI:38.0-66.2%) for EC-DT and 25.5% (95% CI:13.5-37.5%) for EC-DL (P = 0.0065). Pathological complete response in breast and axilla was 47.9% for EC-DT and 23.5% for EC-DL (P = 0.011). Grade 3-4 toxicity did not differ across treatments, except for diarrhoea (2% in EC-DT vs 13.5% in EC-DL, P = 0.030). Multivariate analyses showed that treatment (P = 0.036) and ER (P = 0.014) were the only predictors of pCR in both groups. Conclusion: EC-DT exhibited higher efficacy and lower toxicity than EC-DL. Of the different biomarkers studied, only the absence of ER expression was associated with increased pCR.