Oxidative stress is implicated in the pathogenesis of asthma, and clinical studies show an imbalance in the level of oxidants to the level of antioxidants in subjects with asthma. Aldehydes and glutathione are examples of biomarkers of oxidant-induced damage and antioxidant status in asthma, respectively. In the study, we applied analytical techniques based on liquid chromatography for the assessment of aldehydes and glutathione in the exhaled breath condensate of children with asthma and in control subjects without asthma. Twelve subjects with asthma were evaluated at exacerbation and after 5 days of therapy with prednisone. At exacerbation, malondialdehyde levels were higher in patients with asthma (30.2 ± 2.4 nM) than in control subjects (19.4 ± 1.9 nM, p = 0.002) and were reduced after steroid therapy (18.5 ± 1.6 nM, p = 0.001). At exacerbation, glutathione levels were lower in subjects with asthma (5.96 ± 0.6 nM) than in control subjects (14.1 ± 0.8 nM, p < 0.0001) and were increased after the therapy (8.44 ± 1.2 nM, p = 0.04). Malondialdehyde and glutathione both in subjects with asthma and control subjects were negatively correlated (r = −0.5, p = 0.001). The study shows that aldehydes and glutathione are detectable in the exhaled breath condensate of children with asthma and healthy children and that their levels are modified during asthma exacerbation and after a 5-day course of therapy with oral prednisone.