Lung function was measured in 832 children. Airway symptoms were determined by questionnaires. Urinary levels of a degradation product of lung tissue, hydroxyproline, were measured. Exposure to nitrogen dioxide was determined by measurements inside homes and by personal measurements. Exposure to tobacco smoke was estimated from questions about smoking habits. From the study it is concluded, among other things, that air pollution within homes by NO2 and tobacco smoke leads to small, but statistically significant, effects on lung function and respiratory symptoms in children.