Aim: The purposes of our study were to compare the clinical outcomes of human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer (mBC) patients with or without trastuzumab treatment and HER2-negative patients, and to explore factors that might predict the survival benefit associated with trastuzumab treatment in HER2-positive breast cancer. Patients and Methods: A total of 421 patients with mBC were analyzed in this retrospective study. All patients had first-line chemotherapy with or without trastuzumab. They were classified into 3 groups according to their HER2 status and trastuzumab treatment: HER2-positive mBC patients with or without trastuzumab treatment and HER2-negative patents. Results: Trastuzumab administration in HER2-positive mBC patients significantly prolonged overall survival (33 vs. 26 months; P = 0.003) and led to a 49.8% reduction in death risk. In the subgroup analysis, HER2-positive patients with hormone receptor (HR)-negative status (29 vs. 17 months; P = 0.000) or visceral metastasis (30 vs. 21 months; P = 0.000) had more survival benefit when treated with trastuzumab. Conclusions: Trastuzumab administration significantly improved the overall survival in HER2-positive mBC patients, who gained a prognosis comparable to that of patients with HER2-negative disease. HR status and metastasis site might be important surrogate makers that predict survival benefit from trastuzumab-based treatment. (C) 2018 S. Karger GmbH, Freiburg