Oxidative stress is believed to play an important role in the pathophysiology of asthma. Recently discovered F2-isoprostanes, of which 8-iso-PGF₂α is the most well-known isomer, have emerged as the most reliable marker of in vivo oxidative stress. The aim of this study was to examine 8-iso-PGF₂α as a biomarker of oxidative stress in mild asthma in relation to endogenous and dietary antioxidant protection. Total (free and esterified) plasma 8-iso-PGF₂α, plasma dietary antioxidants (vitamins E and C,β-carotene, Zn, and Se), and erythrocyte antioxidant enzyme activities (glutathione peroxidase and superoxide dismutase) were measured in 15 mild asthmatics and 15 age-and sex-matched controls. Total plasma 8-iso-PGF₂α levels [median (quartile 1-quartile 3)] were significantly increased in the sthmatics [213 pg/mL (122–455) vs. 139 pg/mL (109–174), P=0.042]. The 8-iso-PGF₂α levels were found to be associated with clinical asthma severity (P=0.044) and inhaled corticosteroid use (P=0.027) in asthmatics. No differences were observed in the plasma dietary antioxidant vitamins. The asthmatics had significantly lower plasma levels of Zn (P=0.027) and Se (P=0.006). Plasma Se correlated negatively with 8-iso-PGF₂α (r=−0.725, P=0.002). No differences between the groups were observed for glutathione peroxidase or superoxide dismutase, however, superoxide dismutase activity was negatively associated with asthma severity (P=0.042). In conclusion, oxidative stress is increased in mild asthmatics, as reflected by increased plasma levels of 8-iso-PGF₂α and a deficiency in plasma Zn and Se. The isoprostane 8-iso-PGF₂α may provide a useful tool in intervention studies aimed at improving clinical status in asthma.