The measurement of exhaled nitric oxide concentrations [NO] may provide a simple, noninvasive means for measuring airway inflammation. However, several measurement conditions may influence exhaled NO levels, and ambient NO may be one of these. We measured exhaled NO levels in 47 stable asthmatic children age 5 to 17 years and in 47 healthy children, gender and age matched. Exhaled [NO] in expired air was measured by a tidal breathing method with a chemiluminescence analyzer, sampling at the expiratory side of the mouthpiece. NO steady-state levels were recorded. In order to keep the soft palate closed and avoid nasal contamination, the breathing circuit had a restrictor providing an expiratory pressure of 3-4 cm H2O at the mouthpiece. To evaluate the effect of [NO] in ambient air, measurements were randomly performed by breathing ambient air or NO-free air from a closed circuit. Breathing NO-free air, exhaled [NO] in asthmatics (mean +/- SEM) was 23.7 +/- 1.4 ppb, significantly higher (P < 0.001) than in healthy controls (8.7 +/- 0.4 ppb). Exhaled NO concentrations measured during ambient air breathing were higher (49 +/- 4.6 ppb, P < 0.001) than when breathing NO-free air (23.7 +/- 1.4 ppb) and were significantly correlated (r = 0.89, P < 0.001) with atmospheric concentrations of NO (range 3-430 ppb). These findings show that 1) exhaled [NO] values of asthmatic children are significantly higher than in healthy controls, and 2) atmospheric NO levels critically influence the measurement of exhaled [NO]. Therefore, using a tidal breathing method the inhalation of NO-free air during the test is recommended.