ERBB2 and breast cancer: This real-world study (clinicaltrials.gov identifier: NCT03099200) aimed to assess how living in each stage of HER2-positive BC treatment (patients with early BC currently receiving adjuvant treatment; patients with early BC who have completed adjuvant parenteral therapy; and patients with metastatic BC) impacts directly on patients’ HRQoL and productivity, and indirectly on society in terms of cost of lost productivity, to help quantify the wider benefit of developing new interventions which delay or prevent the metastatic recurrence of BC.