The aim of this clinical trial was to investigate the toxicity and immunological responses of personalized peptide vaccination for cytokine-refractory metastatic renal cell carcinoma patients. Patients were confirmed to be human leukocyte antigen (HLA)-A24 or HLA-A2 positive and had histologically confirmed renal cell carcinoma. Ten patients were enrolled in the present study. The peptides to be administered were determined based on the presence of peptide-specific cytotoxic T lymphocyte precursors in peripheral blood mononuclear cells (PBMC) and peptide-specific IgG in the plasma of cancer patients. Patients received subcutaneous injections of four different peptides (3 mg/peptide) every 2 weeks. Vaccinations were well tolerated without any major adverse events. A minimal increase in peptide-specific interferon-γ production in postvaccination PBMC was observed, regardless of higher levels of cytotoxic T lymphocyte activity in prevaccination PBMC. In contrast, an increase in peptide-specific IgG levels of postvaccination (sixth) plasma was observed in the majority of patients. After progression, five patients received interleukin-2 therapy and continuous vaccination, with survival of 31, 25, 23, 17, and 15 months, but interleukin-2 did not impede humoral responses boosted by the vaccination. These results encourage further clinical trials of personalized peptide vaccinations. (Cancer Sci 2007; 98: 1965–1968)