Woo J, Oh SJ, Song JY, et al. - The present study was conducted to investigate the chemotherapy response of very young patients with estrogen receptor (ER)-positive breast cancer (BC) by pathologic complete response (pCR) after neoadjuvant chemotherapy excluding the effect of endocrine treatment to the extent possible. Between 1992 and 2013, researchers obtained individual patient data from the Korean Breast Cancer Society. They included a total of 1,048 ER-positive and 797 ER-negative patients aged < 50 years who had been treated with neoadjuvant chemotherapy. pCR rate was compared between patients aged < 35 years with ER-positive tumors and the other groups. The results showed that chemotherapy response based on pCR was not better in young patients (< 35 years) with ER-positive BC than in older premenopausal patients with non-metastatic ER-positive BC. In ER-positive BC, young age cannot be a predictive factor of response to neoadjuvant chemotherapy. Different biological characteristics such as a high proliferative index should be recognized.