The aim of this study was to determine whether outdoor nitrogen dioxide (NO 2 ) was associated with the prevalence of asthma and respiratory symptoms.In eight nonurban communities, 843 children resident for a minimum of 2 yrs were studied. Since industrial sources of air pollution were at least 20 km away from the study communities, NO 2 was considered to primarily indicate trafficrelated air pollution. NO 2 was recorded at central monitors, and the 3 yr mean exposure was calculated. Asthma and respiratory symptoms were assessed according to the International Study on Asthma and Allergy in Childhood.Prevalence of asthma at some time ("ever asthma") was associated with longterm NO 2 . In parallel with increasing levels of NO 2 (community specific 3 yr mean 6.0-17.0 parts per billion (ppb)), asthma prevalence was 2.5, 1.4, 1.6, 2.3, 3.4, 3.6, 7.6 and 8.5%, respectively (p=0.002 for trend). The prevalence odds ratios (PORs) for "ever asthma", following adjustment for gender, age, parental education, passive smoke exposure, type of indoor heating, and parental asthma, were 1.28 (95% confidence interval (95% CI) 0.20-7.98), 2.14 (95% CI 0.40-11.3) and 5.81 (95% CI 1.27-26.5), when each of two communities with low, regular and high NO 2 , respectively, were compared with the two communities with very low NO 2 . For symptoms "wheeze" (adjusted PORs for increased NO 2 : 1.47, 1.23 and 2.27) and "cough apart from colds" (adjusted PORs for increased NO 2 : 1.49, 1.93 and 2.07), a similar trend was seen.In this study a significant relationship was observed between traffic-related nitrogen dioxide and the prevalence of asthma and symptoms. Whether this association is causal has to be tested in longitudinal studies.