We have previously demonstrated that mutant vaccinia viruses target tumors in vivo after systemic delivery, and they have potential as vectors for tumor-directed gene therapy. We hypothesized that hyperthermia may augment vaccinia delivery to tumors after systemic injection, as hyperthermia increases the permeability of the endothelial vasculature to nanoparticles. In our in vitro experiments, we have shown that hyperthermia does not alter tumor cells‘ susceptibility to the intrinsic cytopathogenicity of the vaccinia virus compared with normothermic controls. Hyperthermia also does not change the viral infectivity or the level of viral marker gene expression when compared with normothermia. In an in vitro model of endothelial cell monolayer permeability, we have demonstrated that hyperthermia increases the permeability of the monolayer to vaccinia virus and that this phenomenon is completely reversible. In vivo we have demonstrated that the tumors that were treated with systemic vaccinia under conditions of hyperthermia (41.5 degrees C for 30 min) had significantly higher levels of vaccinia marker gene activity (>100-fold) than those treated under normothermic conditions (p < 0.05) and that this effect was specific to tumor. We also demonstrated that mice with 1 cm subcutaneous tumors treated with a systemically delivered, conditionally replicating vaccinia under conditions of hyperthermia had complete tumor regression in 50% and significantly improved antitumor response, compared with normothermic viral-treated controls (mean tumor volume of 110 mm(3) vs 3169 mm(3), 13 days after treatment) and compared with hyperthermic, nonvirally treated control animals (p < 0.0001). Regional hyperthermia improves vaccinia targeting to tumors, and thereby enhances the antitumor response.