Primary care physicians often encounter patients with fever of unknown origin and without apparent causes. Recent advances in laboratory medicine have facilitated diagnostic procedures;however, it is still difficult to determine the critical febrile factor at an early stage. We reviewed the medical records of 174 patients who were admitted due to a chief complaint of fever (>37.5℃) to our hospital during the period from 2004 to 2010. The patients were categorized into patients with infection, inflammation, neoplasm and drug-induced fever. Based on the analysis done by category, it was revealed that the patient's age, body temperature and duration of fever were closely related to the final diagnosis. Serum CRP levels were significantly low in the nonbacterial infection group, while serum levels of sIL-2R were high in neoplasm and drug-induced cases. CRP level on admission was weakly but significantly correlated with body temperature, while duration of fever was inversely related to body temperature. The effectiveness of PET-CT and tissue biopsy for diagnosis was considerably high, particularly in the categories of neoplasm and nonspecific inflammation, respectively, though the effectiveness of bacterial culture was low. Thus, a careful review of physical and laboratory information including body temperature, CRP level, duration of fever, gender difference and history of medication is indispensable for diagnosis. Stepwise categorization and disease classification by comprehensive and systemic checkup are very helpful for determining the causes of fever.