PURPOSE: NY-ESO-1, one of the most immunogenic tumor antigens, is expressed in 15% to 25% of metastatic prostate cancers. The immunological and clinical effects of vaccination with recombinant NY-ESO-1 protein combined with CpG as adjuvant were evaluated. EXPERIMENTAL DESIGN: In a phase I clinical study, patients with advanced prostate cancer were vaccinated with recombinant NY-ESO-1 protein (100 μg) mixed with CpG 7909 (2.5 mg) every 3 weeks intradermally for 4 doses. Objectives of the study were the safety of the vaccine and changes of specific humoral and cellular immunological responses to NY-ESO-1 in relation to detectable NY-ESO-1 expression in the individual tumor. RESULTS: All 12 baseline sero-negative patients developed high-titer NY-ESO-1 antibody responses. B-cell epitope mapping identified NY-ESO-1 p91-110 to be recognized most frequently by vaccine-induced antibodies. Two patients developed significant antibody titers against the adjuvant CpG. NY-ESO-1-specific CD4+ and/or CD8+ T-cell responses were induced in 9 patients (69%). Five of these 9 patients did not express NY-ESO-1 in the autologous tumor. Postvaccine CD8+ T-cell clones recognized and lyzed HLA-matched tumor cell lines in an antigen-specific manner. CONCLUSION: Our data provide clear evidence for the capacity of NY-ESO-1 protein/CpG vaccine to induce integrated antigen-specific immune responses in vivo and to efficiently prime CD8+ T-cell responses in NY-ESO-1 antigen-negative patients. Our results may also support further clinical vaccination protocols with NY-ESO-1 protein not only focused on the treatment of existing cancer, but also to prevent further development of NY-ESO-1 positive cancers in vivo.