Although trastuzumab biosimilars have been approved based on clinical studies on their use as monotherapy or in combination with chemotherapy, clinical studies on their combination with pertuzumab are lacking. Data on the efficacy and safety of this combination are scarce. We evaluated the efficacy and safety of trastuzumab biosimilars in combination with pertuzumab. Progression-free survival was 10.5 months(95% confidence interval[CI]: 3.3-16.3)for a reference biological product and 8.7 months(2.1-not applicable)for biosimilars with a hazard ratio of 0.96(95%CI: 0.29-3.13, p=0.94); however, no statistically significant difference was observed. The incidence of adverse events was not significantly different between the reference biological product and biosimilars, and no increase was observed for any adverse events after switching to the biosimilars. The results of this study verify that a combination of trastuzumab biosimilars with pertuzumab is sufficiently effective and safe in clinical practice.