Episodes of lower respiratory illnesses (LRIs) in the first years of life have been associated with recurrent wheeze in studies of high-risk and community-based cohorts. Respiratory syncytial virus (RSV) is an agent especially associated with severe cases of bronchiolitis affecting young infants in winter months and has a typical seasonal pattern. Data from the Children's Respiratory Study from Arizona and a hospital-based Swedish study have been interpreted as evidence that severe RSV bronchiolitis is associated with a 30-40% likelihood of subsequent asthma. Other respiratory viruses, especially Rhinovirus, have been identified to be importantly associated with recurrent wheeze in children at risk for asthma. A case-control study of palivizumab given in the first year of life to preterm infants has shown a 50% reduction in the occurrence of recurrent wheeze even after controlling for potential confounding variables. Prospective trials with anti-viral strategies, including potential new vaccines, should give us better understanding of the role of viral infections in early life in the causation of childhood asthma.