Tumor-associated macrophages are a prominent component of lung cancer stroma and contribute to tumor progression. The protein V-set and Ig domain-containing 4 (VSIG4), a novel B7 family-related macrophage protein that has the capacity to inhibit T-cell activation, has a potential role in the development of lung cancer. In this study, 10 human non-small-cell lung cancer specimens were collected and immunohistochemically analyzed for VSIG4 expression. Results showed massive VSIG4(+) cell infiltration throughout the samples. Immunofluorescent double staining showed that VSIG4 was present on CD68(+) macrophages, but absent from CD3(+) T cells, CD31(+) endothelial cells, and CK-18(+) epithelial cells. Moreover, VSIG4 was coexpressed on B7-H1(+) and B7-H3(+) cells in these tumor specimens. Transfection of the VSIG4 gene into 293FT cells demonstrated that the VSIG4 signal could inhibit cocultured CD4(+) and CD8(+) T-cell proliferation and cytokine (IL-2 and IFN-γ) production in vitro. Interestingly, in a murine tumor model induced by Lewis lung carcinoma cell line, we found that tumors grown in VSIG4-deficient (VSIG4(-/-)) mice were significantly smaller than those found in wild-type littermates. All of these results demonstrate that macrophage-associated VSIG4 is an activator that facilitates lung carcinoma development. Specific targeting of VSIG4 may prove to be a novel, efficacious strategy for the treatment of this carcinoma.