Background:Despite antibiotic treatment, the mortality of severe community-acquired pneumonia (CAP), especially in patients with severe comorbidity, remains high. Innate defense mechanisms including polymorphonuclear neutrophil (PMN) activation and survival, orchestrated by cytokines, are primarily responsible for the elimination of bacterial organisms from the alveolus. Objectives: The aim of this study was to evaluate the effect of granulocyte colony-stimulating factor (G-CSF) on PMN activation, apoptosis and cytokine response in patients with CAP. Methods: Patients received a single dose of G-CSF (1 × 300 or 480 µg s.c.) prior to standard antibiotic treatment (n = 8) or standard treatment only (n = 8). Apoptosis rate and expression of CD11b, CD66b, CD64 and CD114 surface molecules on systemic PMN were assessedusing fluorescence-activated cell sorter analysis. Levels of the interleukin-1 receptor antagonist (IL-1RA), the soluble tumor necrosis factor receptor inhibitor (sTNF-p55) and G-CSF were measured by ELISA. Results: In the treatment group, 12 h after G-CSF application, neutrophil count increased, neutrophil activation marker CD11b was stimulated (CD11b: 48.6 ± 9.7 vs. 71.2 ± 17.7, p < 0.01), neutrophil apoptosis decreased (apoptosis: 1.36 ± 0.27 vs. 0.2 ± 0.12%, p < 0.01) and the concentration of IL-1RA and sTNF-p55 increased (IL-1RA 136.4 ± 72.2 vs. 340.1 ± 194.6 ng/ml, p < 0.01; sTNF-p55 382 ± 4,243 vs. 632 ± 4,714 ng/ml, p < 0.01; control group nonsignificant). These effects were not seen in the control group. Conclusions: The application of a single dose of G-CSF in patients with CAP caused a prolonged survival and increased activation of neutrophils combined with a sustained release of anti-inflammatory cytokines.