Exacerbations of chronic obstructive pulmonary disease (COPD) cause morbidity, hospital admissions, and mortality, and strongly influence health-related quality of life. Some patients are prone to frequent exacerbations, which are associated with considerable physiologic deterioration and increased airway inflammation. About half of COPD exacerbations are caused or triggered primarily by bacterial and viral infections (colds, especially from rhinovirus), but air pollution can contribute to the beginning of an exacerbation. Type 1 exacerbations involve increased dyspnea, sputum volume, and sputum purulence; Type 2 exacerbations involve any two of the latter symptoms, and Type 3 exacerbations involve one of those symptoms combined with cough, wheeze, or symptoms of an upper respiratory tract infection. Exacerbations are more common than previously believed (2.5-3 exacerbations per year); many exacerbations are treated in the community and not associated with hospital admission. We found that about half of exacerbations were unreported by the patients, despite considerable encouragement to do so, and, instead, were only diagnosed from patients' diary cards. COPD patients are accustomed to frequent symptom changes, and this may explain their tendency to underreport exacerbations. COPD patients tend to be anxious and depressed about the disease and some might not seek treatment. At the beginning of an exacerbation physiologic changes such as decreases in peak flow and forced expiratory volume in the first second (FEV(1)) are usually small and therefore are not useful in predicting exacerbations, but larger decreases in peak flow are associated with dyspnea and the presence of symptomatic upper-respiratory viral infection. More pronounced physiologic changes during exacerbation are related to longer exacerbation recovery time. Dyspnea, common colds, sore throat, and cough increase significantly during prodrome, indicating that respiratory viruses are important exacerbation triggers. However, the prodrome is relatively short and not useful in predicting onset. As colds are associated with longer and more severe exacerbations, a COPD patient who develops a cold should be considered for early therapy. Physiologic recovery after an exacerbation is often incomplete, which decreases health-related quality of life and resistance to future exacerbations, so it is important to identify COPD patients who suffer frequent exacerbations and to convince them to take precautions to minimize the risk of colds and other exacerbation triggers. Exacerbation frequency may vary with the severity of the COPD. Exacerbation frequency may or may not increase with the severity of the COPD. As the COPD progresses, exacerbations tend to have more symptoms and take longer to recover from. Twenty-five to fifty percent of COPD patients suffer lower airway bacteria colonization, which is related to the severity of COPD and cigarette smoking and which begins a cycle of epithelial cell damage, impaired mucociliary clearance, mucus hypersecretion, increased submucosal vascular leakage, and inflammatory cell infiltration. Elevated sputum interleukin-8 levels are associated with higher bacterial load and faster FEV(1) decline; the bacteria increase airway inflammation in the stable patient, which may accelerate disease progression. A 2-week course of oral corticosteroids is as beneficial as an 8-week course, with fewer adverse effects, and might extend the time until the next exacerbation. Antibiotics have some efficacy in treating exacerbations. Exacerbation frequency increases with progressive airflow obstruction; so patients with chronic respiratory failure are particularly susceptible to exacerbation.