Bronchial responsiveness assessed by histamine bronchial challenge testing in 22 children with chronic stable asthma was compared with the number of inflammatory cells per milliliter of broncholavage fluid obtained by fiberoptic bronchoscopy. Hyperresponsiveness was closely correlated with increased counts of eosinophils and macrophages and with the ratio of eosinophils to macrophages. There was no correlation of neutrophil or lymphocyte counts with bronchial hyperresponsiveness and none of the cell types was correlated with airway obstruction. Our findings support the hypotheses that macrophages may be important in the development of bronchial hyperresponsiveness in children with asthma and that they may modulate bronchial responsiveness both directly and by recruitment of eosinophils.