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Background: It has long been assumed that the development of childhood asthma is related to exposure to environmental chemicals, but it has thus far not been possible to unequivocally establish this suspected relationship using individual studies. Moreover, studies of children have been scanty and unreliable due to the large diversity of research environments and subject cohorts. The aim of the current study was to clarify this relationship for one factor by means of a meta-analysis of studies investigating the influence of NO2 exposure on symptomatology of childhood asthma. Methods: Two electronic databases (MEDLINE and EMBASE ) were searched for literature on relationships between environmental chemical exposure and development of childhood asthma using the MeSH terms 'nitrogen dioxide' and 'asthma'. This was done according to the MOOSE guidelines for meta-analyses of observational studies. Results: A total of 130 papers were retrieved, of which 12 met the selection criteria. These papers described observational studies from seven countries. Study subjects were 97 932 ordinary children aged 0-18 years. Using random model analysis, the odds ratio (OR) for asthma development due to an increment of 10 p.p.b. NO2 was 1.135 with a 95% confidence interval [CI] of 1.031-1.251 ( P= 0.01), while the OR for wheezing symptoms was 1.052 with a 95%CI of 1.020-1.085 ( P= 0.001). It is therefore evident that NO2 exposure does influence the development of asthma in ordinary children. Conclusions: Exposure to NO2 in the air significantly influences the development of childhood asthma and symptoms of wheezing. [ABSTRACT FROM AUTHOR]