The measurement of exhaled NO has excited considerable interest, as it may provide a simple noninvasive means of measuring airways inflammation. There is now persuasive evidence that levels of NO are increased in association with airway inflammation and are decreased with anti-inflammatory treatment. Correlation of exhaled NO with more direct measurements of inflammation in the airways, such as induced sputum, bronchoalveolar lavage and bronchial biopsies, is needed. The great advantage of exhaled NO is that the measurement is completely noninvasive. It can, therefore, be performed repeatedly, and also in children and patients with severe airflow obstruction, where more invasive techniques are not possible. However, the measurement is not specific, and exhaled NO is increased in inflammation due to asthma, bronchiectasis, and respiratory tract infections. This means that absolute values are less important than serial measurements in individual patients. The value of this approach has been demonstrated in asthmatic patients, where the dose of inhaled steroid is changed, resulting in increased levels when the dose is reduced and lower levels when the dose is increased. Because exhaled NO is reduced by anti-inflammatory treatments, it may be of use in monitoring whether therapy is adequate. The technique may also have application in the monitoring of the anti-inflammatory effects of new antiasthma drugs, such as selective phosphodiesterase inhibitors, leukotriene antagonists and synthesis inhibitors, and immunomodulators. Because the measurement is precise and reasonably reproducible, it may facilitate the measurement of dose-response effects with anti-inflammatory treatments, which is difficult at present. The analysers for exhaled NO that are currently available are expensive, but in the future it is likely that technological advances will make it possible to miniaturize these analysers, so that they are portable and may even be used at home in conjunction with peak flow meters. -his may lead to their application in epidemiological research, which may be a useful screening measurement for community studies. Although we have discussed exhaled nitric oxide, other volatile substances may also be detected in exhaled air. Thus, ethane and pentane, which are volatile products of lipid peroxidation, and hydrogen peroxide, may be used to detect oxidant stress in the respiratory tract and may also be useful as markers of inflammation. There is little doubt that this is a rapidly expanding area of research.