We have shown the immunogenicity and safety of synthetic carbohydrate vaccines when conjugated to the carrier keyhole limpet hemocyanin (KLH) and given with the adjuvant, QS-21, in patients with biochemically relapsed prostate cancer. To determine whether immune response could be further enhanced with stimulation by multiple antigens, a hexavalent vaccine was prepared using previously determined doses and administered in a Phase II setting to 30 high-risk patients. The hexavalent vaccine included GM2, Globo H, Lewis(y), glycosylated MUC-1-32mer and Tn and TF in a clustered formation, conjugated to KLH and mixed with QS-21. Eight vaccinations were administered over 13 months. All 30 patients had significant elevations in antibody titers to at least two of the six antigens; 22 patients had increased reactivity with FACS. These serologic responses were lower than that seen previously in patients treated with the respective monovalent vaccines. The reciprocal median combined IgM and IgG antibody titers with ELISA against MUC1, Tn, TF, globo H and GM2 for these 30 patients were 640, 80, 120, 40 and 0, compared to 1280, 640, 1280, 320 and 160 seen in patients receiving individual monovalent vaccines. This hexavalent vaccine of synthetic "self" antigens broke immunologic tolerance against two or more antigens in all 30 vaccinated patients, was safe, but antibody titers against several of the antigens were lower than those seen in individual monovalent trials. No impact on PSA slope was detected. We address the relevance of the multivalent approach for prostate cancer treatment.