Background CD8+ T cell-eliciting vaccines are being investigated in breast cancer patients. Preclinical data showed that trastuzumab increases the susceptibility of tumor cells to lysis by vaccine-generated CD8+ T cells suggesting potential benefit of a combination immunotherapy strategy. The current trial was undertaken to demonstrate the safety of this approach. Methods This study was designed as a dose-escalation trial enrolling clinically disease-free, HLA-A2+ or A3+, HER2-positive breast cancer patients. Patients received six monthly inoculations of GP2+ granulocyte-macrophage colony-stimulating factor (GM-CSF) administered concurrently with standard of care trastuzumab. Local and systemic toxicity as well as left ventricular ejection fraction (LVEF) were monitored. Immunologic responses were assessed in vivo by measuring the local reaction and in vitro using an IFN-γ ELISPOT assay. Results Seventeen disease-free breast cancer patients were vaccinated. There were no dose-limiting or grade 3-5 local or systemic toxicities. The median LVEF was unchanged from baseline after vaccination. Mean local reaction at initial inoculation was 28±10 mm and increased to 68±8 mm at the final inoculation (p<0.01). Mean ELISPOT response to GP2 increased from 47±19 at baseline to 144±60 (p=13) after vaccination. Based on safety and immunologic data, the appropriate dose was determined to be 1000 μg of GP2 + 250 μg of GM-CSF. Conclusion The GP2+GM-CSF vaccine is safe and stimulates an immunologic response when given concurrently with trastuzumab. An ongoing phase II trial is evaluating the efficacy of combining a CD8 T cell-eliciting vaccine with trastuzumab in HER2-positive breast cancer patients. Synopsis Combining trastuzumab with CD8+ T cell-eliciting HER peptide vaccines may provide synergistic immunotherapeutic benefit in breast cancer. This phase I trial demonstrates the GP2 + GM-CSF vaccine given concurrently with trastuzumab is safe and stimulates an immune response.