Respiratory infections are common causes of increased asthma for patients of all ages. Current evidence indicates that viral, and not bacterial, infections are the most important respiratory illnesses which increase the severity of asthma. Of the respiratory viral infections associated with increased asthma, rhinoviruses, i.e. the cause of common colds, have proven to be the virus most often found in association with increased asthma severity. Although the association between rhinovirus infections and asthma is most dramatically illustrated in children, asthma patients of all ages can be affected and the attacks of asthma can be severe. Studies to establish the mechanisms by which rhinoviruses enhance asthma severity have begun to focus on how this virus promotes allergic inflammation. We have found that experimental rhinovirus infections enhance airway responsiveness and, perhaps most importantly, the likelihood that a late allergic reaction will occur to an antigen challenge. Furthermore, using bronchoscopy and segmental antigen challenge, we have found that rhinovirus infections promote mast cell release of histamine and the recruitment of eosinophils to the airways. These data support the concept that rhinovirus infections act to promote allergic inflammation and by this mechanism increase both the likelihood of asthma occurring and the severity of wheezing.