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Background: HLX02 is an approved biosimilar of trastuzumab., Objective: This study aimed to evaluated the efficacy, safety, and immunogenicity of HLX02 compared with reference trastuzumab in patients with human epidermal growth factor receptor 2 (HER2)-positive recurrent or metastatic breast cancer., Patients and Methods: This randomized, double-blind, phase III study was conducted at 89 centers in China, the Philippines, Poland, and Ukraine. Eligible patients were randomized (1:1) to receive HLX02 or European Union (EU)-sourced trastuzumab (initial dose of 8 mg/kg, followed by 6 mg/kg every 3 weeks for up to 12 months) in combination with docetaxel intravenously. The primary endpoint was overall response rate up to week 24 (ORR 24 ). Equivalence was declared if the 95% confidence interval (CI) of difference was within ± 13.5%. Safety and immunogenicity were evaluated in patients who received at least one dose of study medication., Results: Between 11 November 2016 and 10 July 2019, a total of 649 patients were enrolled. The ORR 24 was 71.3 and 71.4% in the HLX02 (n = 324) and EU-trastuzumab (n = 325) groups, with a difference of - 0.1% (95% CI - 7 to 6.9), which fell entirely in the predefined equivalence margins. No statistically significant differences were observed in all secondary efficacy analyses. Safety profiles and immunogenicity were comparable in HLX02 and EU-trastuzumab groups. In total, 98.8% of patients in each group experienced at least one treatment-emergent adverse event (TEAE), 23.8 and 24.9% experienced serious TEAEs, and 0.6% in each group had antidrug antibodies., Conclusions: Among patients with HER2-positive recurrent or metastatic breast cancer, HLX02 demonstrated equivalent efficacy and similar safety and immunogenicity to reference trastuzumab., Clinical Trial Registration: Chinadrugtrials.org CTR20160526 (12 September 2016), ClinicalTrials.gov NCT03084237 (20 March 2017), EudraCT 2016-000206-10 (27 April 2017).