Objective To invesitgate the relationship between 8-isoprostane ( 8-iso-PG) level in exhaled breath condensates ( EBCs) and severity of asthma and explore the role of 8-iso-PG in asthma evaluation and monitoring. Methods Fifty-nine patients with asthma were enrolled. In which 15 cases were acute exacerbation, 13 cases were mild intermittent, 15 cases were mild persistent, and 16 cases were moderate-to-severe persistent. Thirteen healthy volunteers were recruited as control. EBCs were collected using EcoScreen system. The 8-iso-PG levels in EBCs were measured by a specific enzyme immunoassay.The patients with mild intermittent asthma were treated with inhaled corticosteroid ( ICS) for one month and their EBCs were recollected for 8-iso-PG measurement. Results Exhaled 8-iso-PG levels were obviously increased in the patients with acute asthma compared with those chronic asthmatics [ ( 47. 2 ±6. 8) pg/mL vs ( 24. 5 ±12. 0) pg/mL, P lt; 0. 01] . In the chronic persistent asthma, the levels were significantly higher in patients with mild persistent and moderate-to-severe asthma [ ( 17. 9 ±1. 2) pg/mL and ( 39. 7 ±4. 0) pg/mL,P lt; 0. 01] . While 8-iso-PG level did not differ significantly in intermittent asthma [ ( 13. 5 ±1. 1) pg/mL]compared with the control subjects ( P gt; 0. 05 ) . After one-month ICS treatment the 8-iso-PG level in the patients with mild intermittent asthma did not change significantly although the ACT score improved. Conclusions 8-iso-PG levels in EBC are associated with the severity of asthma, implicating 8-iso-PG may be useful in monitoring airway oxidative stress in asthma. ICS treatment is incapable of decreasing the 8-iso-PG, suggesting the ICS has minor impact on oxidative stress.