Alfarsi LH, et al. - Whether the therapeutic benefit from endocrine therapy in early estrogen receptor positive (ER+) breast cancer (BC) patients could be predicted by using KIF18A as a biomarker was assessed. Using the METABRIC dataset, genomic level expression of KIF18A was evaluated to determine its prognostic and predictive value in ER + BC patients (n = 1,506). Furthermore, using a large annotated series of early-stage ER + BC (n = 1,592) with long-term follow-up, KIF18A protein expression was assessed by means of immunohistochemistry. In a subgroup of ER + BC characterized by poor clinical outcome, KIF18A appeared to be a candidate biomarker. Findings suggested prognostic significance of high KIF18A expression to predict poor benefit from endocrine treatment for patients with ER + BC. In these patients, clinician decision on benefit from endocrine therapy could be guided by KIF18A measurement prior to treatment.