RATIONALE: The incidence of allergic disorders is increasing in developed countries and has been associated with reduced exposure to microbes and alterations in the commensal bacterial flora. OBJECTIVES: To ascertain the relevance of commensal bacteria upon the development of an allergic response, we utilized a model of allergic airway inflammation in germ-free (GF) mice that lack any exposure to pathogenic or non-pathogenic microorganisms. METHODS: Allergic airway inflammation was induced in GF, specific pathogen free (SPF) or recolonized mice by sensitization and challenge with ovalbumin (OVA). The resulting cellular infiltrate and cytokine production were measured. MEASUREMENTS AND MAIN RESULTS: Our results show that the total number of infiltrating lymphocytes and eosinophils were elevated in the airways of allergic GF mice as compared to control SPF mice, and that this increase could be reversed by re-colonization of GF mice with the complex commensal flora of SPF mice. Exaggerated airway eosinophilia correlated with increased local production of Th2 associated cytokines, elevated IgE production and an altered number and phenotype of conventional dendritic cells (cDC). Regulatory T cell populations and regulatory cytokine levels were unaltered but GF mice exhibited an increased number of basophils and decreased numbers of alveolar macrophages (AM) and plasmacytoid dendritic cells (pDC). CONCLUSIONS: These data demonstrate that the presence of commensal bacteria is critical for ensuring normal cellular maturation, recruitment and control of allergic airway inflammation.