Inflammation plays a central role in the development and progression of chronic obstructive pulmonary disease (COPD). Host factors, such as proteinases and oxidants, have been implicated in causing tissue damage and amplifying the inflammatory process. During exacerbations of COPD, inflammation, oxidant stress, and many of these proteinases are increased. The role of bacteria in exacerbation remains unclear; however, many bacterial factors, such as proteases, surface proteins, lipopoly(oligo)saccharide, and pyocyanin, may also damage lung tissue either directly or by promoting the host inflammatory response. The interaction between host and bacterial factors is complex and both can cause similar damage, making it difficult to dissect the relative contribution of each factor. There have been few relevant studies in man; however, most of the available evidence relates to the importance of the host response. To resolve this issue, several steps are required. Lung secretions need to be collected from patients in the stable clinical state as well as during well defined exacerbations involving bacteria. The appropriate factors need to be identified in the secretions and require evidence that they are functionally active. Specific abrogation should then change the overall balance within the local environment, after which the administration of specific inhibitors/antagonists can be used to confirm that the factor being studied is central to the inflammatory cascade and subsequent tissue damage in patients. Finally, controlled clinical trials will be required to demonstrate that modifying inflammation influences long-term progression.