[Fever of unknown origin].

Fever of unknown origin is diagnosed when the fever (mostly defined as an elevated body temperature ≥38.3 °C measured by rectal or tympanic route) lasts longer than expected, i.e. 5-10 days after the onset of fever. The search for the cause can be difficult and necessitates the special attention of an experienced general pediatrician in collaboration with specialists in pediatric infectious diseases, rheumatic diseases and oncology, nursing personnel, radiologists and others. In approximately half of the cases an infectious cause is finally found; other causes are primarily inflammatory, malignant and noninflammatory diseases. Individual causes with the imminent threat of a severe course should be treated immediately. For the other cases the diagnostic evaluation is paramount, which is wisely planned and executed with determination and openness. The patient history, physical examination, laboratory and device-based diagnostics, imaging and histological examinations can contribute to the final diagnosis. The parents must be escorted through a period of uncertainty and the child should be comforted wherever possible. Spontaneous recovery is also possible. The probatory administration of antibiotics rarely leads to an improvement. After extensive exclusion of infections and malignancies and increasing suffering from the fever itself, prescription of glucocorticoids may be justified in cases of high inflammatory activity, under the suspicion of a hyperergic state and after detailed informed consent. The management of fever of unknown origin is one of the greatest challenges in pediatrics.