The causality of observed associations between air pollution and respiratory health in children is still subject to debate. If reduced air pollution exposure resulted in improved respiratory health of children, this would argue in favor of a causal relation. We investigated whether a rather moderate decline of air pollution levels in the 1990s in Switzerland was associated with a reduction in respiratory symptoms and diseases in school children. In nine Swiss communities, 9,591 children participated in cross-sectional health assessments between 1992 and 2001. Their parents completed identical questionnaires on health status and covariates. We assigned to each child an estimate of regional particles with an aerodynamic diameter < 10 microg/m3 (PM10) and determined change in PM10 since the first survey. Adjusted for socioeconomic, health-related, and indoor factors, declining PM10 was associated in logistic regression models with declining prevalence of chronic cough [odds ratio (OR) per 10-microg/m3 decline = 0.65, 95% confidence interval (CI), 0.54-0.79], bronchitis (OR = 0.66; 95% CI, 0.55-0.80), common cold (OR = 0.78; 95% CI, 0.68-0.89), nocturnal dry cough (OR = 0.70; 95% CI, 0.60-0.83), and conjunctivitis symptoms (OR = 0.81; 95% CI, 0.70-0.95). Changes in prevalence of sneezing during pollen season, asthma, and hay fever were not associated with the PM10 reduction. Our findings show that the reduction of air pollution exposures contributes to improved respiratory health in children. No threshold of adverse effects of PM10 was apparent because we observed the beneficial effects for relatively small changes of rather moderate air pollution levels. Current air pollution levels in Switzerland still exceed limit values of the Swiss Clean Air Act; thus, children's health can be improved further.