Mucin 1, encoded by the MUC1 gene, is a tumor-associated antigen expressed on the surface of breast cancer cells. It would be of interest to see whether there is a naturally existing T cell immune response against mucin epitopes in cancer patients. Materials and Methods: Using tetramer and interferon gamma assays, the immune response to one MUC1 peptide epitope in the peripheral blood of breast cancer patients was quantified. The data were compared with the clinical course of the patients. Results: CD8+ T cells capable of recognizing the HLA-A*0201-restricted STAPPVHNV epitope were detected in 9 of 19 patients with a frequency ranging 0.01?0.082%. No significant difference was found between the occurrence of epitope-specific CD8+ T cells of patients with progressive disease and disease-free patients. However, all patients with stable disease showed a specific immune response, including both patients with the highest frequency. Conclusions: The results of this study provide further evidence that a natural specific cellular immune response against this mucin epitope exists in breast cancer patients.