Background. We are conducting clinical vaccine trials with the HER2/neu peptide, E75, in patients with breast cancer. The purpose of this study was to demonstrate clonal expansion of E75-specific CD8+ T cells and to identify intra- and interantigenic epitope spreading. Method. Pre- and postvaccination peripheral blood leukocyte samples (24 node positive [NP] and 20 node negative [NN]) from 44 vaccinated patients were analyzed. HLA-A2:Ig dimer molecules were loaded with the HER2 peptides, E75 or GP2, and were used with anti-TcR and CD8 antibodies to stain peripheral blood leukocyte immediately ex vivo and were analyzed with flow cytometry. In 8 randomly selected patients, dimers were loaded with the folate binding protein peptide E41 to evaluate for interantigenic epitope spreading. Results. All patients with NP and 95% of the patients with NN showed E75-specific clonal expansion. Patients with NN showed more robust expansion. All patients with NP and 85% of the patients with NN showed evidence of intra-antigenic epitope that was spreading to GP2. However, patients with NN showed only moderate expansion to this subdominant epitope, which was not included in the immunizing mix. The degree of HER2/neu expression and disease stage impacted the ability to expand clonally E75- and GP2-specific CD8+ T cells. Evidence of interantigenic epitope spreading to E41 was shown in 63% of the patients who were tested. Conclusion. Our data provide evidence for the induction of intra- and interantigenic epitope spreading that results from a single HER2/neu peptide vaccine even in early staged patients. The ability to raise immunity to multiple tumor antigens depends on both the degree of HER2/neu expression and the extent of disease. Epitope spreading is an essential element for the success of a peptide vaccine strategy.