Vaccination of patients with cancer using dendritic cells (DCs) was shown to be effective for B-cell lymphoma and malignant melanoma. Here we provide evidence that patients with advanced breast and ovarian cancer can be efficiently vaccinated with autologous DCs pulsed with HER-2/neu– or MUC1-derived peptides. Ten patients were included in this pilot study. The DC vaccinations were well tolerated with no side effects. In 5 of 10 patients, peptide-specific cytotoxic T lymphocytes (CTLs) could be detected in the peripheral blood using both intracellular IFN-γ staining and ⁵¹Cr-release assays. The major CTL response in vivo was induced with the HER-2/neu–derived E75 and the MUC1-derived M1.2 peptide, which lasted for more than 6 months, suggesting that these peptides might be immunodominant. In addition, in one patient vaccinated with the MUC1-derived peptides, CEA- and MAGE-3 peptide-specific T-cell responses were detected after several vaccinations. In a second patient immunized with the HER-2/neu peptides, MUC1-specific T lymphocytes were induced after 7 immunizations, suggesting that antigen spreading in vivo might occur after successful immunization with a single tumor antigen. Our results show that vaccination of DCs pulsed with a single tumor antigen may induce immunologic responses in patients with breast and ovarian cancer. This study may be relevant to the design of future clinical trials of other peptide-based vaccines.