This study conducted a retrospective analysis of 215 patients diagnosed with fever of unknown origin (FUO) between January 2009 and December 2010 at the Chinese PLA General Hospital. The patients were followed up for 18 to 36 months. The most common causes of FUO were infectious diseases (IDs) (42.3%), followed by connective tissue diseases (CTDs) (32.1%), miscellaneous (MI) (10.7%), and neoplasm (N) (6.5%). IDs, CTDs, and N were the most frequent causes, with tuberculosis, adult onset Still disease (AOSD), and non-Hodgkin lymphoma being the most common specific conditions. The study also identified two special types of fever: regularly intermittent fever associated with urinary infections and irregularly intermittent fever associated with infective endocarditis. The fatality rate during the follow-up was 2.8%. The findings suggest that IDs remain the most common cause of FUO, and specific clinical features, such as the types of fever, can aid in diagnosis.This study conducted a retrospective analysis of 215 patients diagnosed with fever of unknown origin (FUO) between January 2009 and December 2010 at the Chinese PLA General Hospital. The patients were followed up for 18 to 36 months. The most common causes of FUO were infectious diseases (IDs) (42.3%), followed by connective tissue diseases (CTDs) (32.1%), miscellaneous (MI) (10.7%), and neoplasm (N) (6.5%). IDs, CTDs, and N were the most frequent causes, with tuberculosis, adult onset Still disease (AOSD), and non-Hodgkin lymphoma being the most common specific conditions. The study also identified two special types of fever: regularly intermittent fever associated with urinary infections and irregularly intermittent fever associated with infective endocarditis. The fatality rate during the follow-up was 2.8%. The findings suggest that IDs remain the most common cause of FUO, and specific clinical features, such as the types of fever, can aid in diagnosis.