The morphology and sequential evolution of the pulmonary lesions in respiratory distress syndrome have been reviewed. Reasons have been presented to support the conclusion that these lesions are in the main produced by toxic effects of oxygen rather than the underlying disease process which initiated the episode of respiratory failure. The mechanism of oxygen toxicity has been briefly reviewed. The implication of these considerations is that in any patient with acute respiratory impairment, oxygen should be used as therapy only if it is required to prevent death from hypoxia and should be used in the lowest possible effective dose. As the patient improves and arterial oxygen tension rises this dose should be promptly reduced.