Chronic obstructive lung disease (COLD) includes emphysema, chronic airways obstruction, and chronic bronchitis but not asthma. COLD mortality and morbidity rates are higher in men than women, in whites than blacks, and they increase with increasing age and with decreasing socioeconomic status. Death rates for COLD are approximately 10 times higher in cigarette smokers than in nonsmokers, and about 80% of COLD deaths in the United States are attributable to cigarette smoking. Among smokers, the quantity and duration of cigarette use are strongly related to mortality and morbidity, but susceptibility varies among individuals. Incidence of COLD is related inversely to pulmonary function and possibly to increased bronchial reactivity. Alpha 1-antiprotease deficiency, respiratory infections and symptoms, lean body build, and exposures to occupational hazards or air pollution have been associated with increased risks of COLD in epidemiologic and clinical studies. Susceptible individuals can reduce their risks of clinical disease by stopping smoking. Clinical trials are underway to determine whether bronchodilators reduce the rates of decline in pulmonary function.