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Fever of unknown origin (FUO) is defined as a temperature higher than 38.3°C on several occasions and lasting longer than 3 weeks, with a diagnosis that remains uncertain after 1 week of investigation.A systematic review was performed to develop evidence-based recommendations for the diagnostic workup of FUO. MEDLINE database was searched (January 1966 to December 2000) to identify articles related to FUO. Articles were included if the patient population met the criteria for FUO and they addressed the natural history, prognosis, or spectrum of disease or evaluated a diagnostic test in FUO. The quality of retrieved articles was rated as “good,” “fair,” or “poor,” and sensitivity, specificity, and diagnostic yield of tests were calculated. Recommendations were made in accordance with the strength of evidence.The prevalence of FUO in hospitalized patients is reported to be 2.9%. Eleven studies indicate that the spectrum of disease includes “no diagnosis” (19%), infections (28%), inflammatory diseases (21%), and malignancies (17%). Deep vein thrombosis (3%) and temporal arteritis in the elderly (16%–17%) were important considerations. Four good natural history studies indicate that most patients with undiagnosed FUO recover spontaneously (51%–100%). One fair-quality study suggested a high specificity (99%) for the diagnosis of endocarditis in FUO by applying the Duke criteria. One fair-quality study showed that computed tomographic scanning of the abdomen had a diagnostic yield of 19%. Ten studies of nuclear imaging revealed that technetium was the most promising isotope, showing a high specificity (94%), albeit low sensitivity (40%–75%) (2 fair-quality studies). Two fair-quality studies showed liver biopsy to have a high diagnostic yield (14%–17%), but with risk of harm (0.009%–0.12% death). Empiric bone marrow cultures showed a low diagnostic yield of 0% to 2% (2 fair-quality articles).Diagnosis of FUO may be assisted by the Duke criteria for endocarditis, computed tomographic scan of the abdomen, nuclear scanning with a technetium-based isotope, and liver biopsy (fair to good evidence). Routine bone marrow cultures are not recommended.