Lung oxidative stress (OS) was explored in resting and in exercising subjects exposed to moderate and high altitude. Exhaled breath condensate (EBC) was collected under field conditions in male high-competition mountain bikers performing a maximal cycloergometric exercise at 670 m and at 2,160 m, as well as, in male soldiers climbing up to 6,125 m in Northern Chile. Malondialdehyde concentration [MDA] was measured by high-performance liquid chromatography in EBC and in serum samples. Hydrogen peroxide concentration [H2O2] was analysed in EBC according to the spectrophotometric FOX2 assay. [MDA] in EBC of bikers did not change while exercising at 670 m, but increased from 30.0 ± 8.0 to 50.0 ± 11.0 nmol l-1 (P < 0.05) at 2,160 m. Concomitantly, [MDA] in serum and [H2O2] in EBC remained constant. On the other hand, in mountaineering soldiers, [H2O2] in EBC under resting conditions increased from 0.30 ± 0.12 μmol l-1 at 670 m to 1.14 ± 0.29 μmol l-1 immediately on return from the mountain. Three days later, [H2O2] in EBC (0.93 ± 0.23 μmol l-1) continued to be elevated (P < 0.05). [MDA] in EBC increased from 71 ± 16 nmol l-1 at 670 m to 128 ± 26 nmol l-1 at 3,000 m (P < 0.05). Changes of [H2O2] in EBC while ascending from 670 m up to 3,000 m inversely correlated with concomitant variations in HbO2 saturation (r = -0.48, P < 0.05). AMS score evaluated at 5,000 m directly correlated with changes of [MDA] in EBC occurring while the subjects moved from 670 to 3,000 m (r = 0.51, P < 0.05). Lung OS may constitute a pathogenic factor in AMS.