Levels of reactive free radicals are elevated in the airway during asthmatic exacerbations, but their roles in the pathophysiology of asthma remain unclear. We have identified subsets of myeloid-derived suppressor-like cells as key sources of nitric oxide and superoxide in the lungs of mice with evolving experimental allergic airway inflammation and established these cells as master regulators of the airway inflammatory response. The profiles of free radicals they produced depended on expression of inducible nitric oxide synthase (iNOS), arginase, and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. These radicals controlled the pro-and anti-inflammatory potential of these cells, and also regulated the reciprocal pattern of their infiltration into the lung. The nitric oxide-producing cells were Ly-6C + Ly-6G - and they downmodulated T-cell activation, recruited T reg cells, and dramatically downregulated antigen-induced airway hyperresponsiveness. The superoxide-producing cells were Ly-6C - Ly-6G + and they expressed proinflammatory activities, exacerbating airway hyperresponsiveness in a superoxide-dependent fashion. A smaller population of Ly-6C + Ly-6G + cells also suppressed T-cell responses, but in an iNOS-and arginase-independent fashion. These regulatory myeloid cells represent important targets for asthma therapy. © 2011 Society for Mucosal Immunology.