Knowledge about the etiology of fever of unknown origin (FUO) has been changed under the influence of new advances in diagnostic techniques in both adulthood and childhood. Seventy-seven patients with the diagnosis of FUO were evaluated retrospectively. Forty-six (60%) of the patients were male and 31 (40%) were female, with ages ranging from 4 months to 16 years (mean: 4.5 years). Physical findings were absolutely normal in 33 (42.9%) patients, and the most common findings were hepatosplenomegaly (15.5%) and lymphadenopathy (15.5%). The etiologies were determined in 69 patients with FUO. The most common diagnoses were infectious diseases (50.7%), malignancy (14.4%), collagen vascular disorders (7.2%), and miscellaneous conditions (27.5%). With the development of diagnostic tools, the etiologies in a considerable number of patients with FUO were diagnosed. A detailed history and physical examination are required for accurate diagnosis, and if indicated, invasive procedures should be instituted.