PURPOSE Human epidermal growth factor receptor 2 (HER2) overexpression is seen in 4%-16% of biliary tract cancers (BTCs). We aimed to evaluate the clinical activity of gemcitabine-cisplatin (GC) plus anti-HER2 antibody trastuzumab as initial treatment in HER2-positive BTCs. METHODS This study was an investigator-initiated, open-label, single-arm, multi-institutional, phase II trial in adult patients with HER2-positive (defined as immunohistochemistry [IHC] 3+ or IHC 2+ and fluorescent in situ hybridization–positive), treatment-naïve BTCs. The primary end point of the study was 6-month progression-free survival (PFS). Next-generation sequencing was performed on tissue samples to evaluate mutational status. RESULTS From March 2020 to August 2022, of the 876 screened patients, 118 (13.4%) were found to have HER2-positive status, of whom 90 were enrolled in the study. Most patients had GBC (n = 96; 96%) with two or more sites of metastatic disease (n = 70; 78%). With a median follow-up of