Respiration is normally accompanied by repetitive stretching and relaxation of the bronchial walls. However, bronchial obstruction typically emerging in asthma and COPD significantly increases the effect of mechanical deformation on the airway epithelial lining causing mechanical stress. Obstructive lung pathology is also usually characterized by chronic airway inflammation and ROS production. Oxidative stress arising during the natural course of asthma and COPD has multiple effects on the progression of these diseases due to aggravated synthesis of pro-inflammatory mediators, mucus hypersecretion, increased permeability of the epithelial barrier and fibrosis. The aim of this study was to investigate the effect of experimentally induced mechanical stress in the airways on the production of 8-isoprostane, a marker of oxidative stress, in asthma and COPD patients. The study included 75 asthma patients and 42 COPD patients with mean age of 47.0±1.47 years. Mechanical stress was induced during the test with 5-min isocapnic (5% CO2) inspiratory resistive breathing with load at 45% of maximum inspiratory pressure. Lung function was measured by standard spirometry and body plethysmography just before and after the test. We accepted the changes (Δ) of FEV1 ≥ 10% and sGaw ≥ 20% as indicative for significant airway response. 8-isoprostane level was quantified by ELISA in exhaled breath condensate collected during 15 min before and after the experimental mechanical stress. The airway responses measured by spirometry and body plethysmography were not concordant: 14% were considered significant only by ΔFEV1 while 36% were significant solely according to ΔsGaw. Besides, there was no correlation between ΔFEV1 and ΔsGaw. The studied patients with ΔsGaw ≥20% or ≤-20% did not differ in baseline (18.0 (13.2-26.4) pg/ml vs. 18.8 (11.9-24.2) pg/ml, respectively, p=0.87) and post-test (18.2 (11.4-22.4) pg/ml vs. 19.6 (15.1-26.7) pg/ml, respectively, p=0.31) concentrations of 8-isoprostane. However, the difference in the relative change of 8-isoprostane level (Δ8-iso) was significant between the groups (-18.7 (-31.7-3.64)% vs. 8.5 (-11.6-30.0)%, p=0.02). In general, the patients with increase in sGaw ≥20% had decrease in 8-isoprostane level. On the contrary, in the patients with significant reduction of sGaw the production of 8-isoprostane was rather elevated in response to the mechanical stress. Increase in 8-isoprostane by ≥20% was observed in 45.5% of patients with ΔsGaw ≤-20%, in 32.8% of patients with low amplitude response (-20%≤ΔsGaw≤20%) and only in 13.8% of patients with ΔsGaw ≥20% (p=0.04). In addition, Δ8-iso was negatively correlated with ΔsGaw (R=-0.18, p=0.05). No significant associations were observed between 8-isoprostane levels and ΔFEV1. In summary, we established that about 30% of asthma and COPD patients respond to inspiratory resisting breathing with significant increase of 8-isoprostane level. This reaction is most common in patients with reduction of airway conductance (sGaw) after the test but not dependent on ΔFEV1.