The influence of indoor nitrogen-dioxide exposure on respiratory symptoms of schoolchildren was investigated in a case-control study. The election method used was useful in obtaining symptomatic children, but insufficient in defining cases and controls without additional information. No relationship between indoor NO2 and respiratory symptoms was found. Bias may have been present, especially because of the high mobility of the study population. Attempts to estimate historical exposure were inaccurate. Therefore the results do not exclude that an association between indoor NO2 and respiratory symptoms exists.