Using sensitive diagnostic methodologies, epidemiologic studies during the past decade have allowed the identification of human rhinoviruses (RVs), generally recognized as "common cold viruses," as major asthma precipitants. This association was further established by evaluating the impact of RV infection in airway obstruction and inflammation during naturally acquired or experimentally induced RV colds. There is now strong evidence that RVs can infect and propagate not only in the upper but also in the lower airways. Bronchial and pulmonary epithelia infected by RVs are rich sources of inflammatory mediators, which may initiate or augment airway inflammation and obstruction. Furthermore, in an atopic environment, responses to the virus are skewed by and toward an "atopic," Th2-like balance, which may further enhance inflammation and exacerbate asthma.