Oncolytic adenoviruses are being tested as biological cancer therapeutics. Ar6pAE2fF (E2F vector) contains the E2F-1 promoter to regulate the expression of the E1a gene in cells with a disregulated retinoblastoma pathway. Ar6pAE2fmGmF (E2F-GM vector) includes the murine granulocyte-macrophage colony-stimulating factor (GM-CSF) transgene to enhance anti-tumor activity. Both vectors selectively killed human tumor cells in vitro. The E2F-GM vector expressed biologically active murine GM-CSF in vitro and GM-CSF was detected for several days in serum and tumor extracts following injections of established human xenograft tumors. In vivo, both vectors showed significant dose-dependent anti-tumor responses. The E2F-GM vector elicited greater efficacy compared to the E2F vector, demonstrating that GM-CSF enhanced the anti-tumor activity, even in immunodeficient nude mice. Histological analysis showed that both vectors induced necrosis and mononuclear cell infiltration, but only the E2F-GM vector resulted in eosinophil infiltration. Vector replication in vivo was demonstrated. The data showed that intratumoral injection of a GM-CSF-armed oncolytic vector induced potent anti-tumor responses in xenograft tumor models, likely as the result of both oncolytic vector activity and the induction of GM-CSF-mediated inflammation and innate immunity.