We briefly consider some examples illustrating the pathophysiologic roles of oxidants in lung injury: pathologic changes induced by hyperoxia (exudative alveolar oedema and proliferation of fibroblasts), cell toxicity (especially endothelial cells) and inactivation of alpha-1-proteinase inhibitor (by oxidation of a methionyl residue at the active site of the inhibitor). These suggest the possibility of a beneficial role for antioxidants in many types of lung injury.