Cigarette smoking has significant detrimental effects on both the structure and function of the lung; it is the single most important risk factor for the development of COPD. Uncertainty remains concerning the mechanisms by which smokers develop obstructive lung disease. It is speculated, however, that an imbalance between proteolytic and antiproteolytic forces in the lung or an increase in heightened airways responsiveness is responsible. Population-based studies have documented lower levels of FEV1, accelerated loss of ventilatory function, and increased respiratory symptoms and infections among smokers compared with nonsmokers. Data from both prospective and retrospective studies have consistently shown increased mortality from COPD, pneumonia, and influenza among cigarette smokers compared with nonsmokers.