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Most of the methods of investigating lung diseases have been invasive untilthe discovery that exhaled nitric oxide can be used as a surrogate marker of airway inflammation, particularlyin asthma. Exhaled nitric oxide (NO) is now established as a marker of airway inflammation. It has beenshown to correlate well with eosinophilic asthmatic airway inflammation, and to be able to predict declinein asthma control and airway function. Altered levels of NO are also associated with other inflammatorylung diseases. In addition, polymorphisms of the genes encoding the three nitric oxide synthases areassociated with phenotypic differences associated with lung diseases. Exhaled NO is, however, non-specific.It is therefore of importance that collecting exhaled breath condensate (EBC) has emerged as a potentialtool in the study of pulmonary diseases. The exhaled breath is collected in a cooling system which allowswater vapour to condense. The EBC contains a number of mediators relating to the NO pathway, includingnitrite as a metabolite of nitric oxide, nitrotyrosine, nitrosothiols plus small molecular mediatorsassociated with oxidative stress, including hydrogen ions, and hydrogen peroxide. In addition, reportsare emerging of the detection of larger molecules which not only include leukotrienes, prostaglandins,albumin and other proteins, such as cytokines, but also macromolecules, for example, DNA. EBC is becominga technique which will allow repeated non-invasive sampling from the respiratory tract thus assistingpulmonary research and possibly the monitoring of lung diseases.