Fever of unknown origin (FUO) is a condition in which the fever lasts for 3 weeks and the body temperature is repeatedly > 38.3°C. The diagnosis cannot be confirmed after at least 1 week of complete history taking, physical examination and routine laboratory tests. Fever is caused by the release of prostaglandin E2 from various pyrogenic sources, which acts on the hypothalamus to raise the thermoregulatory point, resulting in a thermogenic effect. Children are also unique in that they have unstable thermoregulation and are susceptible to environmental and psychological factors, such as high temperatures, diet, strenuous exercise, and medications. Therefore, the etiology of FUO is particularly complex and is a difficult problem for clinicians. It requires clear thinking and good diagnostic and differential diagnostic skills in the process of diagnosis and treatment. Hu Bing, Department of Infectious Diseases, Beijing Children’s Hospital