['Allergic asthma is a chronic disease of the conducting airways characterized by T(H)2 inflammation and tissue remodeling after exposure to inhaled allergens. Although the T(H)2 profile is undisputed, the underlying molecular mechanisms leading to this abnormal T(H)2 profile remain largely unclear. MicroRNAs (miRNAs) are short noncoding RNAs that are important regulators of gene expression in the immune system. However, the role of miRNAs, specifically miR-155, in the regulation of allergic airway inflammation is unexplored.', 'We sought to assess the contribution of miR-155 in a mouse model of allergic airway inflammation.', 'To investigate a role for miR-155 in the regulation of allergic inflammation in vivo, we used miR-155 knockout (KO) and wild-type (WT) mice sensitized and exposed to ovalbumin.', 'miR-155 deficiency resulted in diminished eosinophilic inflammation and mucus hypersecretion in the lungs of allergen-sensitized and allergen-challenged mice compared with WT control animals. This was supported by a reduction in T(H)2 cell numbers and airway T(H)2 cytokine levels and complete abrogation of allergen-induced airway eotaxin-2/CCL24 and periostin levels in miR-155 KO mice. Intranasal instillation of eotaxin-2/CCL24 before allergen challenge partially restored airway eosinophilia in miR-155 KO mice, and adoptive transfer of CD4(+) T cells resulted in a similar degree of airway eosinophilia in miR-155 KO and WT mice. Furthermore, the transcription factor PU.1, a negative regulator of T(H)2 cytokine production, was upregulated in the airways of allergen-challenged miR-155 KO mice compared with WT mice.'] Is microRNA-155 essential for T ( H ) 2-mediated allergen-induced eosinophilic inflammation in the lung?