The highly predictable increase in emergency department visits, hospital admissions, and unscheduled physician consultations for childhood asthma in North America every September is uniquely related to school return. Rhinovirus infection is likely the major trigger, initially affecting asthma in school-age children, followed by similar but lesser increases in asthma morbidity in younger children and in adults. Low use of asthma medications during summer may fuel the epidemic, which may be attenuated by the short-term addition of an effective controller therapy.