Airway inflammation in asthma is not measured routinely in clinical practice. Fractional exhaled nitric oxide (FENO), a marker of airway inflammation, is increasingly used as an outcome measure in asthma intervention studies and yet the reproducibility of FENO measurements is unknown.The reproducibility, day-to-day, diurnal variation and perception of standardised FENO measurements were examined in 59 subjects (40 children aged 7-13 yrs and 19 adults aged 18-60 yrs), both healthy (n=30) and with mild (n=29) asthma. FENO was measured on five consecutive days (four measurements on the same day) for adults and twice on the same day for children.The coefficient of reproducibility expressed as the mean pooled standard deviation (n=59, 675 estimations) was 2.11 parts per billion (ppb) and intraclass correlation coefficient was 0.99 in both children and adults. FENO was significantly higher in asthma subjects (32.3 ppb) than in healthy subjects (16.3 ppb). There was no diurnal or day-to-day variation, or a learning effect, as the result of FENO measurements were identical at results of the beginning and at the end of the study.It was concluded that fractional exhaled nitric oxide measurements are simple, reproducible, free from diurnal and day-to-day variation, and acceptable by both healthy and asthmatic adults and children, as a part of their routine visit to a physician. Asthma is an inflammatory disease, yet airway inflammation is not measured directly and routinely in clinical practice [1]. This makes management of asthma difficult, because it is based on indirect measurements of airway inflammation, such as symptoms and lung function. Symptoms may not accurately reflect the extent of underlying inflammation due to differences in perception, and lung function tests may have little room for improvement in mild asthma. None of these parameters is able to distinguish the effect of different doses of inhaled corticosteroids and both may be affected by bronchodilators. The latter is particularly important because of a recent trend towards use of lower doses of inhaled corticosteroids in combination with long-acting b 2 -agonists.Current invasive (bronchoscopy), or semi-invasive (sputum induction) direct methods to measure airway inflammation are difficult to use repeatedly in clinical practice. The use of sputum induction is limited by its pro-inflammatory effect [2], and a considerable bronchospasm has been reported during sputum induction in moderate (14%) and severe (25%) asthma, as a result of the procedure [3].There has been an explosion of research into exhaled nitric oxide (NO) since levels were found to be increased in asthma [1,4]. Standardised [5,6] measurements of fractional exhaled NO (FENO) provide a completely noninvasive means of monitoring airway inflammation and anti-inflammatory treatment in asthma [1], including a dose-dependent onset and duration of action of inhaled corticosteroids [7]. It may be useful in patients using fixed combination inhalers (corticosteroids and longacting b 2 -agon...