Abstract: Fever of unknown origin (FUO) is defined as fever of 38.3℃ or higher, lasting more than 3 weeks, and undiagnosed at 1 week after admission. The etiology of FUO can be divided into infection, tumor, non-infectious inflammatory diseases and miscellaneous. The spectrum of diseases has changed greatly. The proportion of infections and neoplastic diseases decreased, while non-infectious inflammatory diseases and undiagnosed diseases accounted for the increase in the proportion.¹⁸F fluorodeoxyglucose (FDG) can not only accumulate in tumor cells, but also in other infectious or non-infectious inflammatory cells, thus¹⁸F FDG-positron emission tomography/computed tomography (PET/CT) has been used in the etiological diagnosis of FUO. The sensitivity and specificity of PET/CT for FUO are 67%-100% and 33%-97% respectively. While it is difficult to make a comprehensive analysis of these studies due to the distinct FUO classification standard, case source, types of researches and so on. Generally PET/CT can help determine diagnosis in 45%-67% of patients. PET/CT can be recommended for patients with FUO who have adenopathy, low hemoglobin or increased CRP in early stage as an examination used for diagnosis and differential diagnosis.