Carbon monoxide (CO) can be detected in exhaled air and is increased in adult and childhood persistent asthmatic patients. However, little is known about the exhaled CO concentration in episodic childhood asthma. This study aimed to clarify whether measurement of exhaled CO is useful in monitoring disease activity in children with episodic asthma. We measured exhaled CO concentration by modified Micro-Smokerlyzer in 217 elementary school children (132 boys; mean age, 10 +/- 1 (SE) years; range, 9-12 years), in whom 29 had infrequent episodic asthma without current exacerbations. We also measured exhaled CO concentrations in 22 children with episodic asthma (13 boys; mean age, 10 +/- 3 years; range, 8-12 years), who had acute mild asthmatic attacks during examination. In these patients with mild asthmatic attacks, exhaled CO was measured both before and after combination therapy with salbutamol and sodium cromoglycate (SCG) by powered nebulizer. Among 217 schoolchildren, exhaled CO levels in infrequent episodic asthmatic children (1.1 +/- 0.1 parts per million (ppm), n = 29) were not significantly different from those in healthy schoolchildren (1.0 +/- 0.1 ppm, n = 188, P > 0.68). The exhaled CO concentrations during asthma attacks in children with episodic asthma were significantly higher (5.1 +/- 0.4 ppm, n = 22) compared with those in healthy children (P < 0.001) or those in asymptomatic asthmatic children (P < 0.001). The elevated exhaled CO levels were significantly decreased after inhalation therapy of a combination of salbutamol and SCG (3.2 +/- 0.5 ppm, n = 22, P < 0.02). In conclusion, exhaled CO levels were significantly elevated during acute asthma exacerbations, and partially recovered after treatment with beta(2)-agonist and SCG in children with mild episodic asthma. These findings indicate that measurement of exhaled CO might provide another noninvasive measurement of asthma exacerbations that would be suitable for use in children with acute mild episodic asthma.