The recent development of PCR for the diagnosis of respiratory viral infections has permitted studies revealing the importance of virus infections in acute exacerbations of asthma. Several studies implicate rhinovirus as the major virus type in mild and severe wheezing illness in children of all age groups, but particularly over 1 year of age. Rhinoviruses have been shown to replicate in the lower airway, suggesting that virus induced asthma exacerbations result from direct inoculation, spread of the virus from the upper to the lower airway. The importance of RS virus infection in bronchiolitis and wheezing in infants has been reaffirmed. Recent studies using PCR to detect C pneumoniae, suggests a high prevalence of chronic infection in asthmatic children, and that the immune response to this organism may play a pathological role in asthma. These studies now require confirmation with larger carefully controlled studies.