Acute exacerbations of chronic bronchitis reflect increased airway inflammation and are characterised by one or more symptoms of increased sputum production, sputum purulence, and breathlessness. The causes are multifactorial, and bacterial infection is involved in about half of cases. A proportion of patients also have chronic colonization of the bronchial tree between exacerbations, and this may act as a stimulant of airway inflammation. Colonization represents a balance in which compromised host defences limit bacterial numbers but do not eradicate them. The balance is upset during an exacerbation, often due to extraneous factors such as a viral infection or air pollution, leading to increased bacterial numbers and consequently more inflammation. In patients with severe airway damage, infective exacerbations are more likely to occur, and serious consequences may result if baseline lung function is impaired or there are comorbid conditions. In these circumstances, the exacerbation is less likely to resolve spontaneously. Antibiotic treatment benefits patients by achieving bacterial eradication and resolution of the inflammatory response. However, since superficial mucosal infections may resolve spontaneously, there are serious concerns about widespread antibiotic use in patients with more trivial illness. Future studies should include better definition of the type of patients enrolled, improved techniques to determine bacteriological response, and better outcome measures.