This thesis covers three studies. First of all the reproducibility of non invasive biomarkers was assessed and then a cross sectional study of these biomarkers was carried out. Finally a study of the impact of an inhaled anti oxidant on health status and non invasive biomarkers in subjects with COPD was carried out. In 47 subjects, total cell counts and macrophage differential counts were reproducible but not neutrophil and eosinophil differentials. IL-8 and VGEF but not IL-1β and IL-6 demonstrated reproducibility in induced sputum supernatant. Exhaled breath condensate was measured in 24 subjects. 8-Isoprostane but not hydrogen peroxide was reproducible. Exhaled breath condensate was collected in 78 with COPD and 61 controls. Levels of 8-Isoprostane and hydrogen peroxide were measured. No significant differences were seen between the mean levels of these two biomarkers measured in COPD and control groups. COPD ex-smokers from the lowest quartile (Q1) of 8-Isoprostane measured had lower health status and exacerbation frequency compared to the highest quartile (Q4). Lung function was worse in the highest 8-Isoprostane quartile. Hydrogen peroxide levels in EBC did not relate to health status or symptom scores. Fifty-eight subjects with moderate severe COPD participated in a 12 week double blind placebo controlled trial of an inhaled lyseine salt of N-Acetylcysteine. Fewer of the subjects in the low dose treatment arm of the study had exacerbations and demonstrated improvements in terms of diary card reporting of breathlessness when compared with placebo. Trends towards improvement in health status compared with placebo were also noted.