Chronic obstructive pulmonary disease (COPD) is a slowly progressive disorder characterized by airways obstruction that lasts for at least several months. The two major causes of COPD are chronic bronchitis and emphysema. Either disorder may occur with or without airways obstruction, but airways obstruction causes impairment of lung function leading to disability and death. COPD is a major health problem in the United States and throughout the world, consistently ranking among the most common causes of death in the United States. Cigarette smoking is the primary environmental factor that increases the risk of COPD, but other environmental factors have also been implicated. However, despite a well-established role, environmental factors alone do not cause COPD. Symptomatic COPD develops in only 10-20 percent of heavy cigarette smokers, probably those with a genetic susceptibility, although common COPD susceptibility genes have yet to be identified.