[Fever of unknown origin].

Fever of unknown origin (FUO) is defined as sustained unexplained fever despite intensive diagnostic evaluation and represents a particular diagnostic challenge. It can be classified into different categories, e.g. classical, nosocomial, neutropenic and HIV-associated FUO, which is based on the patient-specific clinical and immunological situation. Infections, malignant diseases and non-infectious inflammatory diseases have to be considered as the most important causes of FUO; however, no definitive diagnosis can be established in a substantial number of FUO patients despite an extensive diagnostic work-up. The present review focuses on the important diagnostic aspects as well as therapeutic options in FUO patients.