The authors studied humoral and CD4+ T-cell responses in an HLA-A24+ prostate cancer patient vaccinated with cytotoxic T lymphocyte (CTL)-directed peptides, including a prostate-specific antigen (PSA)248-257 peptide, to understand what kinds of immune responses are elicited in peptide-vaccinated patients. The levels of immunoglobulin G (IgG) reactive to the administered PSA248-257 peptide or the PSA protein were kinetically examined. The level of IgG reactive to the PSA248-257 peptide drastically increased after the peptide vaccination, with a peak after the seventh vaccination, whereas that of IgG reactive to the PSA protein continued to increase throughout the vaccination period. IgG reactive to the PSA protein after the 13th vaccination showed no reactivity to the administered PSA peptides. However, HLA-DRB1*1302-restricted and PSA protein-recognizing TH1-type CD4+ T-cell clone and line, with different specificity, were successfully established from the post-7th and post-13th peripheral blood mononuclear cells, respectively. Both CD4+ T cells produced interferon-gamma in response to naturally processed PSA secreted from prostate cancer cells, whereas their reactivity to the administered PSA248-257 peptide was undetectable or negligible. These findings indicate that vaccination with CTL-directed peptides, including a PSA-derived peptide, was able to elicit and spread humoral and TH1-type immune responses to the PSA protein.