This study examined consecutive patients with unexplained fever (UF) presenting to the ED to define their characteristics and to compare distinctive parameters between admitted and discharged patients. During a 3-month period, all adult patients presenting to the ED with UF were prospectively followed for 1 month. Of 139 patients with UF, 58 patients (42%) were admitted to the hospital, whereas 81 patients (58%) were discharged. Whereas most of the discharged patients had self-limited febrile disease and eventually recovered, the admitted patients had more unresolved fever, serious infections, or systemic diseases and a 5% mortality rate. The admitted patients were older, had more comorbidity, higher leukocyte count, and anemia, but not a higher degree of fever. Older age, comorbidity, leukocytosis, and anemia, but not higher degree of fever, should direct the decision toward admission of a patient with UR (C) 2003 Elsevier Inc. All rights reserved.