Human rhinovirus (HRV) causes the majority of common colds and possibly also asthma exacerbations. Mechanisms linking viruses to changes in airway reactivity are not defined and we hypothesized that changes in endobronchial cell populations may be implicated. Bronchial mucosal biopsies taken before, during and after experimental infections with HRV serotype 16 were examined and histamine reactivity was measured in 17 adult volunteers (6 atopic asthmatics). Biopsies were examined for mast cells, eosinophils, lymphocytes and neutrophils by immunohistochemical techniques. Increases in histamine responsiveness (PC20) were found (p = 0.048), accompanied by an increase in submucosal lymphocytes (p = 0.05). There was a significant increase in epithelial eosinophils with the cold (p = 0.042), and in asthmatics this appeared to persist into convalescence. Rhinoviral common colds are associated with a bronchial mucosal lymphocytic and eosinophilic infiltrate that may be related to changes in airway responsiveness and asthma exacerbations.