Although not conclusive, several lines of evidence suggest that cigarette smoking alters the respiratory tract's ability to defend itself from infection. Some subjects with chronic bronchitis have colonization of the lower respiratory tract with bacteria. Both patients with chronic respiratory disease and healthy smokers appear to have a higher frequency of respiratory infections and an increased severity of symptoms when infected. Children exposed passively to cigarette smoke have higher rates of respiratory illnesses. Yet the marked variability in the incidence of infection in the smoking population suggests that there are subtle factors that predispose some smokers to more risk of infection than others. Cigarette smoking is associated with alterations in mechanisms of the host defense system, even in asymptomatic individuals (summarized in Table 3). Ciliary function is impaired, mucous volume is increased, humoral response to antigens altered, and quantitative and qualitative changes in cellular components occur. Some of these alterations in host defense mechanisms are dose related; others revert to normal after smoking cessation. Yet, it is unknown if one or all of these alterations cause any significant compromise of host defense or if other factors, as yet unidentified, may be important. Answers to these questions await a more thorough elucidation of normal host defense function.