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In cystic fibrosis (CF), there is an imbalance in the oxidant/antioxidant system, leading to oxidative damage. The aim of this study was to assess antioxidant-scavenger deficiencies and lipid peroxide variations in three clinical situations (stable status, acute exacerbation, and after intravenous antibiotic treatment). The objective was also to correlate oxidative stress with age, nutritional status, and respiratory function. The study included prospectively 312 consecutive patients and 53 controls. Antioxidants (vitamin A, vitamin E, carotenoids, and glutathione) and oxidative markers (malondialdehyde and lipid peroxides) were measured in plasma. Regression analyses were performed. Antioxidant levels were lower in CF patients than in controls. These levels decreased during acute exacerbation and increased after antibiotic treatment. Carotenoid levels were not modified by infection or age. Only vitamin A and carotenoid levels were positively correlated to body mass index. Antioxidant levels were correlated to forced expiratory volume at 1 sec. Lipid peroxidation markers were lower in patients than in controls. Their levels decreased during infection, and increased after antibiotic treatment. Impaired lung function was correlated with elevated malondialdehyde levels. In conclusion, this study demonstrates antioxidant deficiency in a very large cohort of CF patients. Carotenoid and vitamin E deficiencies occur early in the course of the disease. Antioxidants decrease with bronchial infection. By contrast, nutritional disorders did not modify antioxidant levels during acute exacerbations. Thus, pulmonary disorders rather than nutritional disorders seem to be essential in the imbalance of the oxidant/antioxidant system. Results concerning glutathione and oxidative-marker levels highlighted the fact that their plasma values do not reflect oxidative stress in the respiratory tract.