Dual human epidermal growth factor receptor 2(HER2)blockade with trastuzumab(H)and pertuzumab(P)combined with docetaxel and carboplatin(TCb)is a standard neoadjuvant therapy for HER2-positive breast cancer patients. We conducted this sub-study using data from the investigator-initiated randomized phase 2 JBCRG-20(Neo-peaks)study to evaluate the safety of simultaneous mixed HP infusion in Japanese patients, as there have been no data to date. A total of 204 patients in groups A-C received TCbHP, TCbHP followed by trastuzumab emtansine(T-DM1)+P, and T-DM1+P, respectively. Of the 103 patients in groups A and B who received H and P by sequential infusion in cycle 1, the 17(median age 59; range 29-69 years)who did not experience an infusion reaction(IF)received these agents as a mixed, single-bag infusion from cycle 2 onwards. No cases of IF were observed, thus 71 mixed doses were safely administered. Administration time was reduced to 60 min from cycle 3 onwards. Furthermore, in the group B patients, mixed HP infusion did not affect their subsequent treatment(i. e. 4 cycles of T-DM1+P). Simultaneous administration of H and P enables a reduced administration time, which would benefit both patients and healthcare providers.