To assess bronchial reactivity in children who have had bronchiolitis, we studied 48 children by challenging them with methacholine nine or ten years after admission to hospital with bronchiolitis. Pulmonary function was also evaluated. Fifty-seven percent of children studied had bronchial hyperreactivity. Thirty-three percent of first-degree relatives of those with a positive MCH challenge had a positive response. There was a significant correlation between the occurrence of a positive MCH challenge and a history of recurrent bronchiolitis. Pulmonary function tests demonstrated lower flow rates in the positive responders. Fourteen children had a history of asthma or wheezing, but this did not appear to be severe or frequent, and few required long-term therapy. There appears to be a strong genetic component in the prevalence of bronchial reactivity in these children. Bronchial hyperreactivity may be a risk factor in the development of COPD.