Free oxygen radicals result from aerobic cellular metabolism; their toxicity is prevented by immediate degradation due to an endless variety of biochemical systems. The nature of these radicals, their cellular production as well as the defence mechanism which oppose their toxic effects are successively and briefly analysed. The potential role of these radicals in the genesis of different lung diseases is still poorly understood. However, certain toxic agents (oxygen, gas pollutants, ionising radiation, toxic products) can act as a whole or at least in part by their intermediaries. The experimental arguments in favour of this hypothesis are reviewed in passing. If the relative importance of the toxic mechanism is still imprecise, free radicals are certainly implicated in pulmonary disease and constitute a new aspect of respiratory patho-physiology.