Tumor necrosis factor (TNF)-alpha is present in the microenvironment of human tumors, including malignant pleural effusion (MPE). Although the cytokine is produced in the pleural cavity by both tumor and host cells, its effects on MPE formation are unknown. In these studies, we sought to determine the role of TNF-alpha in the pathogenesis of MPE and to assess the therapeutic effects of its neutralization in a preclinical model. For this, MPEs were generated in immunocompetent mice using intrapleural injection of mouse lung adenocarcinoma cells. The roles of tumor- and host-derived TNF-alpha were assessed using combined experimentation with TNF-alpha gene-deficient mice and in vivo TNF-alpha neutralization. To expand the scope of preclinical data, TNF-alpha and vascular endothelial growth factor (VEGF) expression were determined in human cancer cell lines and human MPE. In the MPE model, TNF-alpha of host and tumor origin was present. TNF-alpha neutralization significantly limited tumor dissemination, effusion formation, vascular hyperpermeability, TNF-alpha and VEGF expression, and angiogenesis, thereby improving survival. In contrast, these variables were not different between TNF-alpha gene-sufficient and TNF-alpha gene-deficient mice. In mouse cancer cells, TNF-alpha functioned via nuclear factor-kappaB- and neutral sphingomyelinase-dependent pathways to induce TNF-alpha and VEGF, respectively. These results were recapitulated in human cancer cells, and a correlation was detected between TNF-alpha and VEGF content of human MPE. We conclude that tumor-derived TNF-alpha is important in the development of MPE in mice, and provide preclinical evidence supporting the efficacy of TNF-alpha blockade against malignant pleural disease.