Oxidative stress is functionally involved in the pathophysiology of lung diseases including asthma and chronic obstructive pulmonary disease. 8-Isoprostane, which is derived from free radical-catalyzed peroxidation of arachidonic acid, is one of the most reliable biomarkers of oxidative stress. Exhaled breath condensate (EBC) is a completely noninvasive method for collecting airway secretions. We developed a specific and sensitive radioimmunoassay (RIA) that has been applied to the measurement of 8-isoprostane in EBC. This RIA for 8-isoprostane has been validated using high performance liquid chromatography. Measurement of 8-isoprostane in EBC is a useful noninvasive technique for exploring the role of oxidative stress in lung diseases. This technique might provide important insights into the understanding of the clinical pharmacology of antioxidants and might be useful for monitoring the effects of pharmacological therapy.