BACKGROUND The inadequacy of systemic treatments of advanced colorectal cancer has aroused interest in biologic therapy. Recent animal models have demonstrated the efficacy and safety of a recombinant vaccine that contains vaccinia and the gene for carcinoembryonic antigen (rV-CEA). METHODS A phase I clinical trial of rV-CEA was conducted to assess vaccine toxicities, the maximum tolerated dosage, resulting immune activities, and tumour responses. A dose-escalation protocol was devised for three concentrations. Six patients per dosage were each to receive three vaccinations. RESULTS Seventeen patients with advanced colorectal cancer received a total of 44 vaccinations. Mild local and systemic reactions-comparable to those seen with vaccinia alone-were observed and were typically associated with the first vaccination. No significant complications or deaths were caused by the rV-CEA. In particular, no autoimmune colitis developed, nor did leukopenia occur, despite some homology between CEA and leukocyte antigens. All three vaccine concentrations were equally well tolerated. Most patients demonstrated tumor progression by clinical and radiographic parameters and by CEA levels. Immune assays are pending. CONCLUSIONS This phase I trial demonstrated the safety of rV-CEA in patients with advanced colorectal cancer. Future clinical studies are warranted and will likely be influenced by investigations of the immune responses to the vaccine.