To study the association between upper and lower respiratory viral infections and acute exacerbations of asthma in schoolchildren in the community. Community based 13 month longitudinal study using diary card respiratory symptom and peak expiratory flow monitoring to allow early sampling for viruses. 108 Children aged 9-11 years who had reported wheeze or cough, or both, in a questionnaire. Southampton and surrounding community. 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. 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. This study supports the hypothesis that upper respiratory viral infections are associated with 80-85% of asthma exacerbations in school age children.