ESRRB and breast cancer: Only 4 patients (0.2%) died of a cardiac cause.<h4>Conclusions and relevance</h4>In this secondary analysis of a randomized clinical trial, 1-year vs 9-week adjuvant trastuzumab was associated with improved DFS among patients with ERRB2-positive breast cancer receiving chemotherapy, but there was no significant difference in OS between the groups.<h4>Trial registration</h4>ClinicalTrials.gov Identifier: NCT00593697.