To test the hypothesis that reactive oxygen species could be associated to the lower airway disorders occurring in horses suffering from recurrent airway obstruction (RAO), indices of oxidative stress were studied in blood and pulmonary epithelium lining fluid in 5 RAO horses either in clinical remission or 24 h after the onset of a crisis of bronchospasm and in 5 healthy horses. Venous blood and bronchoalveolar lavage fluid (BALF) samples were collected and analysed for reduced glutathione (GSH), oxidised glutathione (GSSG), total glutathione (TGSH), glutathione redox ratio (GRR) in blood haemolysate and pulmonary epithelium lining fluid (PELF). The haemolysate concentrations of GSH, GSSG, TGSH and GRR were similar in the 3 groups. The PELF glutathione status was significantly different in the RAO horses in acute crisis compared to healthy horses, indicating the occurrence of an oxidative stress. When RAO horses were in crisis their GSH and TGSH remained unchanged but their GSSG and GRR were significantly increased compared to the remission. These results support the hypothesis that oxidative stress is associated with lower airway disorders occurring in horses suffering from RAO.