Objective To investigate the causes and features of fever of unknown origin(FUO).Methods This study enrolled 89 patients who fullfilled the criteria of FUO and were admitted in China-Japan Friendship hospital from October 2005 to May 2008.Results The final diagnosis were made in 80 cases(89.9%).Etiologies of FUO were as follows:infections,41 cases(46.1%);rheumatic disease,15 cases(16.9%);neoplasm,18 cases(20.2%)(hematological malignancy,17 cases),miscellaneous diseases 6 cases(6.7%),unknown etiology,9 cases(10.1%).Conclusion Infection is the main cause of FUO;rheumatic disease and neoplasm are also important causes of FUO.Hematological malignancy is the most common neoplasm related to FUO.Detailed history contributes greatly to FUO diagnosis.