Objectives: Immuno-gene therapy of mesothelioma with an adenovirus encoding interferon-β mediated strong antitumor responses in murine models with low but not high tumor burden. Our goals were to determine the mechanisms responsible for this loss of efficacy and to test the hypothesis that the combination of preoperative adenovirus encoding interferon-β and surgical resection would be effective in treating bulky tumors. Methods: Flank tumors of a mouse mesothelioma cell line were treated with adenovirus encoding interferon-β or adenoviral vector encoding the bacterial protein β-galactosidase. Cytotoxic T lymphocytes and tumor infiltration by T lymphocytes were measured. Tumors were surgically excised 72 hours later and tumor cells were injected in the contralateral flank to create a model of a metastatic focus. Tumor-free survival and distant metastatic disease were assessed. Results: Immuno-gene therapy effectively treated small tumors (<200 mm3) but did not reduce the size of large (>800 mm 3) flank tumors. Although treatment with adenovirus encoding interferon-β resulted in the generation of tumor-neutralizing splenocytes in large tumors, the number of T cells visualized within the tumors was minimal. Tumors treated with adenovirus encoding interferon-β (versus adenoviral vector encoding the bacterial protein β-galactosidase or phosphate-buffered saline solution) prior to debulking increased long-term tumor-free survival and resulted in two- to sixfold smaller foci of implanted tumor cells at 2 weeks postoperatively. Conclusions: The use of adenovirus encoding interferon-β or surgical debulking alone is ineffective in treating large tumors, but combining preoperative adenovirus encoding interferon-β and surgical debulking significantly reduces tumor recurrence and improves long-term tumor-free survival. We postulate that adenovirus encoding interferon-β amplifies the cytotoxic T-lymphocyte antitumor response, allowing elimination of residual tumor cells.