Asthma is a major health problem worldwide. The measurement of fractional exhaled nitric oxide (FENO) has been established as a valuable non invasive and simple tool in the diagnosis of asthma and may also act as a useful surrogate inflammatory marker on which to base treatment decisions in asthma management algorithms. The measurement is useful also in other respiratory diseases. Current methods of measuring FENO include on line measurements by heavy duty expensive analyzers which are not widely and easily available. Off line measurements of breath samples which can be analysed later may be a simple solution. We hypothesize tha toff line measurements of NO will be as reliable and valid as those measured on-line