Invasive pulmonary aspergillosis is a common and devastating complication of immunosuppression, whose incidence has increased dramatically in tandem with the increase in the number of immunocompromised patients. Given the role of TNF-α in other pulmonary infections, we hypothesized that TNF-α is an important proximal signal in murine invasive pulmonary aspergillosis. Intratracheal challenge with Aspergillus fumigatus conidia in both neutropenic (cyclophosphamide-treated) and nonneutropenic BALB/c mice resulted in the time-dependent increase in lung TNF-α levels, which correlated with the histologic development of a patchy, peribronchial infiltration of mononuclear and polymorphonuclear cells. Ab-mediated neutralization of TNF-α resulted in an increase in mortality in both normal and cyclophosphamide-treated animals, which was associated with increased lung fungal burden as determined by histology and as quantified by chitin content. Depletion of TNF-α resulted in a reduced lung neutrophil influx in both normal and cyclophosphamide-treated animals, which occurred in association with a decrease in lung levels of the CXC chemokine, macrophage inflammatory protein-2 and the CC chemokines macrophage inflammatory protein-1α and JE. In cyclophosphamide-treated animals, intratracheal administration of a TNF-α agonist peptide (TNF 70–80) 3 days before, but not concomitant with, the administration of Aspergillus conidia resulted in improved survival from 9% in control mice to 55% in TNF 70–80-treated animals. These studies indicate that TNF-α is a critical component of innate immunity in both immunocompromised and immunocompetent hosts, and that pretreatment with a TNF-α agonist peptide in a compartmentalized fashion can significantly enhance resistance to A. fumigatus in neutropenic animals.