Cystic fibrosis (CF) is characterized by considerable oxidative stress. However, it is not known whether oxidative stress is an important feature early in this disease. We have investigated a group of infants and young children with CF to establish whether oxidants are produced in their airways. Bronchoalveolar lavage fluid (BALF) was assayed for myeloperoxidase as a measure of neutrophilic inflammation, and 3-chlorotyrosine as a biomarker of the potent oxidant hypochlorous acid, which is formed by myeloperoxidase. Protein carbonyls were also measured as a nonspecific indicator of reactive oxidant production. Myeloperoxidase and 3-chlorotyrosine levels in BALF from children with CF were 10- and fivefold higher, respectively, than in disease control subjects. There was a strong correlation between myeloperoxidase and 3-chlorotyrosine. Myeloperoxidase levels were fourfold higher in children with infections in their airways. Median protein carbonyls were elevated by only twofold compared with disease control subjects, but some children had extremely high levels of protein oxidation. We conclude that hypochlorous acid is produced early in CF and that it is a candidate for precipitating the fatal decline in lung function associated with this disease. Also, there must be other sourcesof oxidants because protein carbonyls were not related to either inflammation or infection.