Background and objectives: Exhaled breath condensate (EBC) pH has been proposed as a biomarker of airway inflammation and oxidative stress in asthma. Cigarette smoking reduces EBC pH in mild asthma. The effects of smoking on EBC pH in more symptomatic asthmatic patients using inhaled corticosteroids (ICS) are unknown. We aimed to compare EBC pH in asthmatic smokers (AS) and non-smokers (ANS) with moderate to severe disease, who were taking ICS. We also investigated the relationship between EBC pH and biomarkers of airway inflammation and oxidative stress. Methods: AS (n = 18) and ANS (n = 17), who were using ICS, were recruited and EBC pH, sputum inflammatory cell counts and sputum supernatant 8-isoprostane concentrations were measured. Full lung function testing was performed. Results: EBC pH was significantly lower in AS than in ANS (6.91 vs 7.41). In AS there was a significant inverse correlation between EBC pH and 8-isoprostane levels (r = -0.54, P = 0.03). There was no correlation between EBC pH and sputum neutrophil counts. Conclusions: EBC pH appears to be a biomarker of the level of oxidative stress in smokers with moderate to severe asthma. EBC pH may have applications for the longitudinal monitoring of the effects of smoking on the airways of asthmatic patients. © 2009 Asian Pacific Society of Respirology.