The relationship between infections of the respiratory tract and exacerbations of pulmonary symptoms in individuals with asthma is well established on clinical grounds. Patients having an acute attack of asthma often give a history of a "cold" before the onset of the exacerbation. The identification rate of viruses during exacerbations of asthma (10-30%) is much higher than the viral identification rate generally found during asymptomatic periods in asthmatics (3%). The mechanisms whereby upper respiratory viruses might induce or contribute to attacks of asthma are currently unknown: epithelial damage, increased cytokines releasability, mouth breathing.... Influenza vaccination is recommended in patients with chronic pulmonary diseases. However, bronchial hyperreactivity has been reported after influenza vaccination in asthmatics. Reactions to these vaccines may be due to non-immunogenic impurities, which are not present in the more recently developed subunit vaccines. In spite of the lack of double-blind studies between subunit and killed influenza virus vaccines, and because of the potential bad prognosis of influenza infection on airway obstruction, influenza vaccination should be recommended in asthmatics with stable respiratory function but influenza vaccination rate remains low.