Abstract Purpose. This double-blind, randomized, phase III clinical trial evaluated time to progression (TTP) and overall survival in women with metastatic breast cancer (MBC) who received sialyl-TN (STn) keyhole limpet hemocyanin (KLH) vaccine. Secondary endpoints included vaccine safety and immune response. Experimental design. The study population consisted of 1,028 women with MBC across 126 centers who had previously received chemotherapy and had had either a complete or a partial response or no disease progression. All women received one-time i.v. cyclophosphamide (300 mg/m2) 3 days before s.c. injection of 100 μg STn-KLH plus adjuvant (treatment group) or 100 μg KLH plus adjuvant (control group) at weeks 0, 2, 5, and 9. Subsequently, STn-KLH without adjuvant or KLH without adjuvant was then administered monthly for 4 months, and then quarterly until disease progression, without cyclophosphamide. Results. STn-KLH vaccine was well tolerated; patients had mild to moderate injection-site reactions and reversible flu-like symptoms. Week-12 antibody testing revealed high specific IgG titers and a high rate of IgM-to-IgG seroconversion; the median IgG titers in STn-KLH recipients were 320 (anti-ovine submaxillary mucin) and 20,480 (anti-STn), with no detectable antimucin antibodies in the control group. The TTP was 3.4 months in the treatment group and 3.0 months in the control group. The median survival times were 23.1 months and 22.3 months, respectively. Conclusions. Although STn-KLH was well tolerated in this largest to date metastatic breast cancer vaccine trial, no overall benefit in TTP or survival was observed. Lessons were learned for future vaccine study designs.