Tumor necrosis factor-α (TNF-α) plays an important role in innate immunity. Recent in vitro studies have shown that TNF-α may also serve as a growth factor for some bacteria. We examined the physiologic relevance of this phenomenon both in vitro and in vivo. Recombinant mouse TNF-α increased in vitro proliferation of Escherichia coli but not Pseudomonas aeruginosa in a concentration-dependent manner, and this effect was attenuated by anti–TNF-α antibodies. However, in vivo, TNF-α gene-deficient (TNF-α^−/−) mice showed higher mortality than wild-type (TNF-α^+/+) mice after inoculation of intranasal bacteria. An impaired bacterial clearance in TNF-α^−/− mice was associated with decreased systemic concentrations of chemokine macrophage inflammatory protein-2, reduced pulmonary neutrophil recruitment, and depressed expression of neutrophil CD11b and CD16/CD32, suggesting that the effect of TNF-α on E. coli growth was outweighed by the recruited neutrophils. We also demonstrated that neutropenic TNF-α^+/+ mice had approximately 100-fold higher E. coli counts in their lungs than TNF-α^−/− mice, although survival rates in both groups were similar. We conclude that TNF-α augments E. coli growth in vitro and in vivo. However, in vivo, this effect becomes only apparent in neutropenic animals. The relevance of these findings for immune compromised patients remains to be investigated.