Key PointsQuestionDo outcomes differ for patients with hormone receptor (HR)–positive metastatic breast cancer (mBC) with lowERBB2expression compared with those with noERBB2expression when treated with targeted therapies in combination to endocrine therapy (ET)?FindingsIn this cohort study of 1585 patients, the outcomes for patients with HR-positive mBC with lowERBB2expression were similar to those for patients with noERBB2expression when treated with targeted therapies in combination with ET. This concordance applied similarly to both ductal and lobular histologic subtypes.MeaningLowERBB2status did not have a significant association with prognosis in patients with HR-positive mBC treated with targeted therapies plus ET.