Background: TCGA analysis revealed that somatic EGFR, ERBB2, ERBB3 and ERBB4 mutations (ERBB family mutations) occur alone or co-occur with somatic PIK3CA mutations in 19% of HER2-positive breast cancers. As ERBB family mutations can activate the AKT pathway it is likely that they have similar canonical signalling effects to PI3K pathway mutations. Therefore we have investigated their combined impact on response to neoadjuvant HER2-targeted therapies. Methods: Baseline breast tumour biopsies were available from 83 patients with HER2-positive early breast cancer who were enrolled to the phase 2 TCHL neoadjuvant study assessing TCH (docetaxel, carboplatin, trastuzumab) versus TCHL (TCH and Lapatinib), each for 6 cycles. We used mass spectrometry-based genotyping to determine the frequency and outcome implications of 105 ERBB family and PIK3CA single nucleotide mutations. Results: PIK3CA mutations were detected in 19 patient tumour samples (22.9%), while EGFR and ERBB3 mutations were each detected in 3 samples (3.6%). ERBB4 mutations were detected in 4 samples (4.8%). EGFR, ERBB3 and ERBB4 mutations co-occurred with PIK3CA mutations in one sample each. Mutation frequency was similar in each treatment arm (29.7% in TCH arm and 34.6% in TCHL arm), and was not influenced by ER status (29.6% in ER-negative patients, 31.8% in ER-positive patients) or PR status (30.8% in PR-negative patients, 31.3% in PR-positive patients). We analysed the correlation between tumour PIK3CA/ERBB family mutation status and pathological complete response (pCR), and found no significant difference in pCR rates between patients with wild-type (WT) tumours and patients whose tumours carried mutations (38.7% vs 54.5%, respectively; p = 0.312). However, in the TCHL (but not the TCH) group the pCR rate was higher for patients with PIK3CA/ERBB family mutated tumours than for patients with WT tumours (58.3% vs 14.3%; 95% CI, 1.038 to 16.6%; p = 0.0375). Conclusions: Our results indicate that patients who receive neoadjuvant TCHL and have PIK3CA/ERBB family mutated tumours are more likely to have a pCR relative to patients with WT tumours.