Nitric oxide is a molecule that under normal conditions is synthesised from L-arginine, thanks to the action of the so called NOS-c (nitric oxide synthethase constituents) in different cells, in very small amounts. They behave like a neurotransmitter, modulating different vascular functions of the flat muscle in the aerial vias. However, the synthesis of NO can also come about by means of the action of the so called NOS-i (nitric oxide synthethase inducers) whose expression is induced by endotoxins and different pro-inflammatory cytokines. Their activity gives rise to enlargements of an abrupt nature, that are associated to inflammatory conditions. In asthma it has been proven that there is an increase in the ENO figures, which are above the normal amount that the general population have; causing an inflammatory condition of the air way; a basic characteristic of the pathogenesis of asthma, that conditions the obstruction and the hyper-reactivity of the air ways that conclude in defining asthma according to the current concept. Until now, the valuation of the inflammation of the aerial vias is done by serum determinations of other inflammatory markers that are subject to other influences and some of them are not very reliable, apart from being expensive; or by determinations of these markers in induced sputum, or bronchoalveolar ablution. The difficulty of obtain this type of samples in young children means that it is not viable to use this system to assess the inflammation for daily practice. The determination of the ENO in the expired air is carried out by the chemoluminiscence measurement of the synthesis of O2N produced after the NO reacts with the ozone. This is a photochemical reaction and emitys infrared light in proportion to the concentration of the NO in the exhaled air. In the presentation, we will try to analyse the role that the ENO plays on the inflammatory pathology of the respiratory tree; which techniques we can use to measure it; for which reasons the measurement can be altered and finally how it behaves in respiratory allergy. In general the literature on this theme, which is very extensive, shows some defects: there is a disparity in the methods used to collect exhaled air, the pathological situations which determine the ENO are different (for example: patients being treated with inhaled corticoids and people who have never been treated with corticoids) the groups of patients are small and all this together makes it difficult to understand the value of the determination of the ENO in daily practise. But this does not mean that we feel isn't useful, on the contrary: it confirms the need to study the behaviour of the ENO levels in asthma, both in basal situations as well as in relation to the treatment of this illness.