Background: The infusion rate is considered to affect incidence and severity of infusion reactions (IRs) caused by protein formulations. Trastuzumab (TRS) is approved for 90-minute infusion as the initial dose followed by 30-minute infusion with 250 ml saline. In the study, we evaluated the safety of TRS intravenously administered over 30 minutes with 100 ml saline to reduce burden of patients, safety of infusion with 250 ml saline already being established. Materials and Methods: Women with HER2 positive breast cancer, ${geq}18$ years and ${geq}55%$ left ventricular ejection fraction (LVEF), were registered in the study. Patients received 8mg/kg of TRS 250 ml over 90 minutes followed by 6mg/kg of TRS 100ml over 30 minutes in a three-week cycle. Results: A total of 31 patients were recruited, 24 for adjuvant therapy and seven with metastases. The median age was 59 years (range 39 to 82). The total number of TRS doses ranged from 5 to 17 with the median of 15. Mild IR occurred in two patients at the first dose. However, no IR was observed after reducing to 100 ml saline. No decrease of LVEF, increase of serum brain natriuretic peptide or any other adverse events were reported. Conclusions: Intravenous infusion of TRS with 100 ml saline over 30 minutes in breast cancer patients can be considered safe based on results from the study. It can be given on an outpatient basis as with the currently recommended dilution in 250 ml saline.