Recombinant Listeria vaccines induce tumor-specific T-cell responses that eliminate established tumors and prevent metastatic disease in murine cancer models. We used dogs with HER2/neu(+) appendicular osteosarcoma, a well-recognized spontaneous model for pediatric osteosarcoma, to determine whether a highly attenuated, recombinant Listeria monocytogenes expressing a chimeric human HER2/neu fusion protein (ADXS31-164) could safely induce HER2/neu-specific immunity and prevent metastatic disease. Eighteen dogs that underwent limb amputation or salvage surgery and adjuvant chemotherapy were enrolled in a phase I dose escalation clinical trial and received either 2 × 10(8), 5 × 10(8), 1 × 10(9), or 3.3 × 10(9) CFU of ADXS31-164 intravenously every 3 weeks for 3 administrations. Only low-grade, transient toxicities were observed. ADXS31-164 broke peripheral tolerance and induced antigen-specific IFNγ responses against the intracellular domain of HER2/neu in 15 of 18 dogs within 6 months of treatment. Furthermore, ADXS31-164 reduced the incidence of metastatic disease and significantly increased duration of survival time and 1-, 2-, and 3-year survival rates when compared with a historical control group with HER2/neu(+) appendicular osteosarcoma treated with amputation and chemotherapy alone. These findings demonstrate that ADXS31-164 administered in the setting of minimal residual disease can induce HER2/neu-specific immunity and may reduce the incidence of metastatic disease and prolong overall survival in a clinically relevant, spontaneous, large animal model of cancer. These findings, therefore, have important translational relevance for children with osteosarcoma and adults with other HER2/neu(+) cancers. Clin Cancer Res; 22(17); 4380-90. ©2016 AACR.