To examine the relationship between bronchial hyperresponsiveness (BHR) and respiratory symptoms associated with asthma, we studied a sample of 380 schoolchildren on three occasions at 2-yr intervals. The age of the children at the first study was 8-10 yr. Respiratory symptoms history was assessed by questionnaire, BHR was measured by a histamine inhalation test and atopy was assessed by skin-prick tests to 13 allergens. The cumulative prevalence of BHR in this sample was 27%. The severity of BHR was categorized as severe, moderate, mild or slight. The distribution of severe, moderate and mild BHR was similar at each of the studies. At the third study, when the children were aged 12-14 yr, the prevalence of slight BHR decreased. Children with severe or moderate BHR at age 8-10 yr were atopic, reported current symptoms during the 4 yr of the study and had a high prevalence of severe or moderate BHR in later studies. In this group, 87% of children had current respiratory symptoms and 73% were using asthma medication at age 12-14 yr. In children with mild or slight BHR when first studied, the prevalence of atopy, continuing respiratory symptoms and medication use was much lower. We conclude that severe or moderate BHR is an important risk factor for ongoing morbidity and that comparisons of the prevalence of this severity of BHR in populations may be more informative than comparisons of BHR defined by present criteria.