The carotenoids are potent antioxidants with the ability to quench singlet oxygen and other toxic oxygen species. The aim of this pilot study was to investigate the protective effect of beta-carotene on oxidant system in patients with cystic fibrosis (CF) and in patients with bronchiectasis (BE) caused by a reason other than CF. Eighteen children with CF and 15 children with BE followed in the Pediatric Chest Disease Unit of Hacettepe University, and 15 healthy children participated in the study. Compared with the controls, significantly lower plasma levels of beta-carotene were found in the CF group and significantly lower plasma levels of vitamin E in the CF and BE groups. The standardization of carotenoid levels for total cholesterol did not significantly attenuate these differences. In addition, there were significantly higher levels of malondialdehyde (a marker of lipid peroxidation) and tumour necrosis factor-alpha (TNF-alpha) in children with CF and in children with BE than in normal subjects. After 6 mo of beta-carotene supplementation, the plasma levels of beta-carotene and vitamin E increased and the plasma levels of TNF-alpha and malondialdehyde decreased in both groups.