Objective: To investigate which cells are the main source of hydrogen peroxide (H2O2) production in stable asthma and the possible relationship between H2O2 levels on the one hand, and airway inflammation and disease severity on the other. Material and methods: Fifty stable asthmatics with disease severity ranging from mild to moderate were studied. H2O2 was measured in expired breath condensate and its relationship with variables indicating asthma severity [e.g. FEV1% pred, peak expiratory flow rate (PEFR) variability, symptoms score and airway responsiveness to histamine] or airway inflammation [differential cell counts in induced sputum, levels of eosinophil cationic protein (ECP) in sputum supernatant] was assessed. Results: Our results showed that mean (95% CI) concentrations of H2O2 were significantly elevated in patients with asthma compared to the values obtained from control subjects (.67, .56 .77 vs .2, .16 .24 mM, respectively, P<.0001). The difference was primarily due to the significantly increased H2O2 levels observed in moderate asthma patients as compared to those observed in patients with mild persistent or mild intermittent asthma (.95, .76 1.12 μM vs .59, .47 .7, vs .27, .23 .32 μM, respectively, P<.0001). H2O2 levels were positively related to sputum eosinophilia, as well as to ECP concentrations. A positive relationship was also found between H2O2 levels and neutrophil counts in patients with moderate asthma. H2O2 levels were also associated with symptoms score and PEFR variability. There was an inverse relationship between H2O2 levels and FEV1% pred. Further analysis showed that the relationship between H2O2 levels and the examined variables was significant only in moderate asthma patients not receiving inhaled steroids. Conclusions: Eosinophils are the main H2O2 producing cells in asthma of varying severity, whereas neutrophils might also contribute to the much higher H2O2 levels observed in the more severe cases. The predictive value of H2O2 levels as regards disease severity and inflammatory activity is limited and depends on the use of inhaled steroids and the severity of the disease itself. Pneumon 2002, 15(3):263-274.