Cancer immunotherapy has been shown to be effective in isolated instances, but ineffective in most patients. The reason for this inconsistency, and the conditions necessary for successful immunotherapy, are not well understood. Our studies in animal tumor models indicate that the effectiveness of IL-12-based immunotherapy depends upon the presence of a preexisting immune response to the tumor. The presence of T cell infiltrates and IFN-gamma expression in tumor indicate such an immune response. We therefore plan to evaluate the existing immune response in human tumor samples in an effort to identify those patients most likely to respond to therapy with IL-12.