Trastuzumab, a monoclonal antibody, and lapatinib, a tyrosine kinase inhibitor are approved treatments for HER2 positive breast cancer; however, patients that initially respond frequently develop resistance. The aims of this study were to investigate microRNA expression in acquired and/or innate trastuzumab and lapatinib resistant cell lines, and in a cohort of HER2 positive breast cancer patients. Four miRNAs were identified as altered in the acquired trastuzumab resistant cell lines;. miR-221, -222, -224 and -9. miR-221 and -224 are also involved in innate trastuzumab resistance. miR-221, -224, -30e-3p, -550, -92a and -9 are altered in acquired lapatinib resistant cells lines and miR-221 is associated with innate lapatinib resistance. These microRNAs are potential predictive biomarkers of innate and/or acquired resistance to HER2 targeted therapies. All-trans retinoic acid (ATRA) treatment alone or in combination with trastuzumab or lapatinib may overcome resistance in some HER2 positive breast tumours. Co-amplification of retinoic acid receptor alpha (RARα) with HER2 may be a potential biomarker to predict benefit from combined ATRA and HER2 targeted therapies. Over-expression of miR-221 and -222 in the SKBR3-P cell line does not confer resistance to trastuzumab. However, they increase the migratory potential of the SKBR3-P cell line. We identified miR-221, -222 and -9 expression is higher and miR-224 is lower in patients who did not achieve a durable complete response (non-DCR) compared to the DCR cohort. This miRNA profile may be predictive for acquired trastuzumab resistance. This is the f his is the f his is the f his is the first report of the involvement of miR-222, -224 and miR-9 in innate and acquired trastuzumab and/or lapatinib resistance in HER2 positive breast cancer. These miRNAs represent potential biomarkers of resistance and ATRA treatment may be a potential therapeutic strategy to overcome resistance, in some cases.