The aim of the present study is to evaluate the biochemical parameters in blood relevant to oxygen free radicals and antioxidant defenses in children with asthma. A total of 210 asthmatic children, aged 5-18 years, were studied at two different times, once during a severe episode of wheeze (during episode category) and the other after recovery (resting condition). A total of 180 healthy children participated in the study as age and sex matched healthy controls. Superoxide and hydroxyl radical assays were used as a measure of free radical formation. Antioxidant enzymes and free radical scavengers in blood were also assayed. Lipid peroxidation products were assayed in plasma and erythrocytes to evaluate the imbalance (if any) between oxidant (radical) formation and their inactivation. Serum IgE concentrations and peak expiratory flow rate (PEFR) were used as measures of allergic reactions and residual lung capacity, respectively. Excessive production of superoxide and hydroxyl radicals were noted in the blood cells in asthmatics and were correlated to the severity of disease measured as PEFR. Superoxide dismutase and free radical scavengers in blood were significantly lower in asthma, even during resting condition. The present observations endorse the correlation between disease severity and oxygen radical production in asthma subjects. Oxygen metabolites may play a direct or indirect role in the modulation of airway inflammation. Excessive superoxide and hydroxyl radical production may be used as a marker for susceptibility to asthma and for monitoring therapeutic measures.