The recent development of PCR for the diagnosis of respiratory viral infection has highlighted the importance of these infections in acute exacerbations of asthma. Rhinoviruses are important in all age groups, but particularly over 1 yr, while the role of RSV in bronchiolitis and wheezing in infants has been reaffirmed. Recent studies using the same technique for the detection of C. pneumoniae suggest a high prevalence of chronic infection in asthmatic children, and that the immune response to this organism may play a pathological role. These studies now require confirmation with larger carefully controlled studies.