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Acute exacerbations of chronic obstructive pulmonary disease were a poorly defined and understood entity. The application of better study designs and new research methodologies has shed considerable light on the pathogenesis of this common clinical syndrome.Inflammation is an important component of the pathogenesis of chronic obstructive pulmonary disease, and exacerbations probably represent acute increases in airway inflammation brought about by one or more etiological agents. Environmental particulate and gaseous pollutants have been linked in epidemiological studies with increased respiratory symptoms and mortality in chronic obstructive pulmonary disease. Bacterial, viral and atypical pathogens, either alone or in concert, induce the majority of acute exacerbations.A bacterial cause of a substantial proportion of exacerbations is now firmly established by the results of bronchoscopy, molecular epidemiology, immunology and airway inflammation studies. Future research should focus on pathogenic mechanisms and host defence against the microbial pathogens associated with acute exacerbations of chronic obstructive pulmonary disease, in order to develop better treatment and prevention strategies.