There is increasing evidence that bronchial asthma is associated with increased oxidative stress. Reactive oxygen species are produced in each organism as by-products of several essential reactions and can damage biomolecules. Asthma is a complex chronic inflammatory disorder of the airways, with many candidate genes suspected as being important in its development. Glutathione S-transferase (GST) is such a gene due to its role in protection against oxidative stress. In the present study, we examined the hypothesis that increased oxidative stress and polymorphism in the GST-T1 gene are associated with childhood asthma. We found that the amount of sulfhydryl groups significantly decreased and the content of thiobarbituric acid-reactive substances increased in the group of asthmatic children, compared with healthy controls. The GST-T1 null genotype was more frequent among the asthma patients. These results suggest that the GST-T1 null genotype and increased oxidative stress may play a role in the asthma pathogenesis in children.