Fever is one of the most common symptoms in children. In previous studies, infectious disease was the most common cause of pediatric fever of unknown origin (FUO). The aim of this study is to investigate the etiology, clinical characteristics and prognosis of pediatric FUO in 21 century with more diagnostics available and to analyze the factors for certain disease categories.

Among the children under 18 years old who were hospitalized at Samsung Medical Center from January 2000 to December 2014, the patients who met the criteria including fever of ≥38.0℃ for longer than ≥14 days and failure to reach a diagnosis after one week of investigations were included.

Total 100 patients were identified. Confirmed diagnosis was achieved in 57 patients (57%). Among them, infectious diseases (n=19, 19%) were most common, followed by connective tissue diseases (n=15, 15%), necrotizing lymphadenitis (n=8, 8%), and malignancies (n=7, 7%). Children with fever duration over 28 days had a trend for higher frequency of connective tissue diseases (28.3%) except undiagnosed etiology. The symptoms such as arthritis, lymph node enlargement and only fever without other symptoms were significantly related with connective tissue diseases, necrotizing lymphadenitis and undiagnosed respectively ( P <0.001). Ninety-two patients have become afebrile at discharge and 1 patient died (1%).

Almost half of our patients were left without diagnosis. Although it has been known that infectious disease was most common cause of pediatric FUO in the past, undiagnosed portion of FUO have now increased due to development of diagnostic techniques for infectious diseases.