Fever is a common phenomenon in critically ill patients. Ideally, all decisions about treatment of fever would be based on results of well-developed research studies. Instead, some research questions related to treatment of fever have been studied more extensively than others. This article is an evaluation of the usefulness of the research studies related to treatment of fever in the critically ill. Two questions are addressed: when is treatment of fever most beneficial to a critically ill patient, and how is fever most effectively treated in the critically ill? Evidence related to the treatment of fever is evaluated by using the recommendation levels of the research-based practice protocols of the American Association of Critical-Care Nurses.