Fever in elderly persons is only one clinical presentation that can be used to assist the clinician at suspecting a serious disease, such as an infection. Infections, like all other illnesses in the geriatric patient, may occur with a variety of nonspecific, atypical, nonclassic, and unusual manifestations. The clinician caring for elderly patients should be aware of these nonclassical presentations of infections in this age group. Unexplained change in functional capacity, worsening of mental status, weight loss or failure to thrive, weakness and fatigue, falls, and generalized pain are only some of the clues that may aid the clinician in considering infection in elderly persons. Key concepts of fever in older adults are: Fever generally indicates presence of serious infection, most often caused by bacteria. Fever may be absent in 20%-30% of elderly patients harboring a serious infection. Criteria for fever in elderly patients should also include an elevation of body temperature of at least 2 degrees F from baseline values. FUO in elderly persons is caused by infections (30%-35%), CTD (25%-30%), and malignancies (15%-20%) in the majority of cases.