OBJECTIVE--To study the association between upper and lower respiratory viral infections and acute exacerbations of asthma in schoolchildren in the community. DESIGN--Community based 13 month longitudinal study using diary card respiratory symptom and peak expiratory flow monitoring to allow early sampling for viruses. SUBJECTS--108 Children aged 9-11 years who had reported wheeze or cough, or both, in a questionnaire. SETTING--Southampton and surrounding community. MAIN OUTCOME MEASURES--Upper and lower respiratory viral infections detected by polymerase chain reaction or conventional methods, reported exacerbations of asthma, computer identified episodes of respiratory tract symptoms or peak flow reductions. RESULTS--Viruses were detected in 80% of reported episodes of reduced peak expiratory flow, 80% of reported episodes of wheeze, and in 85% of reported episodes of upper respiratory symptoms, cough, wheeze, and a fall in peak expiratory flow. The median duration of reported falls in peak expiratory flow was 14 days, and the median maximum fall in peak expiratory flow was 81 l/min. The most commonly identified virus type was rhinovirus. CONCLUSIONS--This study supports the hypothesis that upper respiratory viral infections are associated with 80-85% of asthma exacerbations in school age children.