The analysis of CF sputum samples confirmed the presence of heterogeneous levels of GGT in sputum. GGT activity correlates with neutrophil elastase and with the products of GGT-mediated GSH catabolism. GSH treatment did significantly reduce sputum levels of inflammatory markers but only in subjects presenting with a regression of inflammation. Differentiating patients with increasing /decreasing GGT activity may discriminate subjects with resolving inflammation, more likely profiting from inhaled GSH, from those with exacerbation of inflammation in which GSH may even produce aggravation of the damage.