This paper reviews accumulated knowledge concerning the role of virus infection in asthmatic bronchitis and describes several studies, carried out in the author\'s department, whose findings have elucidated this and other aspects of its aetiology. Rhinoviruses are associated with asthmatic bronchitis far more frequently than any other viruses, due in large part to their numerous antigenically distinct serotypes. Two host factors appear to be involved in the aetiology of asthmatic bronchitis. Impairment of the defence mechanisms of the lower respiratory tract would permit extension of viral infection into the bronchi, where it causes an inflammatory reaction. Reversible airflow limitation of sufficient magnitude to give rise to wheeze and dyspnoea depends upon the second host factor of bronchial hyperreactivity. Since inflammation is the major component of airflow limitation, beta-adrenoreceptor agonists are of limited value in reducing it, whereas corticosteroids are highly effective. The role of interferon is discussed in relation to prevention and treatment of asthmatic bronchitis.