The attenuated vaccinia virus, modified vaccinia Ankara, has been engineered to deliver the tumor antigen 5T4 (TroVax). TroVax has been evaluated in an open-label phase 2 trial in hormone refractory prostate cancer patients in which the vaccine was administered either alone or in combination with granulocyte macrophage-colony stimulating factor (GM-CSF). The comparative safety and immunologic and clinical efficacy of TroVax alone or in combination with GM-CSF was determined. Twenty-seven patients with metastatic hormone refractory prostate cancer were treated with TroVax alone (n=14) or TroVax+GM-CSF (n=13). 5T4-specific cellular and humoral responses were monitored throughout the study. Clinical responses were assessed by quantifying prostate-specific antigen concentrations and measuring changes in tumor burden by computer-assisted tomography scan. TroVax was well tolerated in all patients with no serious adverse events attributed to vaccination. Of 24 immunologically evaluable patients, all mounted 5T4-specific antibody responses. Periods of disease stabilization from 2 to >10 months were observed. Time to progression was significantly greater in patients who mounted 5T4-specific cellular responses compared with those who did not (5.6 vs. 2.3 mo, respectively). There were no objective clinical responses seen in this study. In this study, the combination of GM-CSF with TroVax showed similar clinical and immunologic responses to TroVax alone. The high frequency of 5T4-specific immune responses and relationship with enhanced time to progression is encouraging and warrants further investigation.