Interleukin (IL)-6 concentrations are positively associated with the severity of pneumonia, and this cytokine is essential to surviving experimental pneumococcal pneumonia. The role that IL-6 plays during pneumonia and its impact during gram-negative bacterial pneumonia remain to be determined. During Escherichia coli pneumonia, IL-6-deficient mice had increased bacterial burdens in their lungs, indicating compromised host defenses. Decreased neutrophil counts in alveolar air spaces, despite normal blood neutrophil counts and survival of emigrated neutrophils, suggested that defective neutrophil recruitment was responsible for exacerbating the infection. Neutrophil recruitment requires nuclear factor (NF)- kappa B, but IL-6 was neither sufficient nor essential to induce NF- kappa B-mediated gene expression in the lungs. In contrast, IL-6 induced the phosphorylation of signal transducer and activator of transcription (STAT) 1 and STAT3 in the lungs, and STAT1 and STAT3 phosphorylation during E. coli pneumonia was decreased by IL-6 deficiency. Thus, IL-6 plays essential roles in activating STAT transcription factors, enhancing neutrophil recruitment, and decreasing bacterial burdens during E. coli pneumonia.