There is growing pressure to demonstrate the value of practice guidelines. We have reviewed the evidence that guidelines for the treatment of community-acquired pneumonia (CAP) change current practices and that the standardization of practices reduces costs and/or improves outcome. The most obvious barrier to implementation of the guidelines is lack of knowledge about their content; equally important are the attitudes and behavior of professionals, patients, and their caregivers. Guidelines may improve the outcome of CAP, provided that there is an association between variations in outcome and some specific processes of care. Conversely, when there is no such relationship, guidelines may reduce the cost of care without having an adverse effect on outcome. The cost-effectiveness of CAP guidelines in an individual hospital depends on the systems that are available to identify patients with CAP and to measure the processes of care. There is good evidence that following the recommendations of the CAP guidelines does improve the cost-effectiveness of care and, therefore, that an audit of CAP may be worth the effort.