To investigate the causes of fever of unknown origin (FUO), we analyzed the clinical data on 142 patients with FUO admitted to our department from January 2002 to December 2003. After various examinations and specific treatment, a definitive diagnosis was reached in 122 cases. Of them, 51 cases (35.9%) were caused by infections, 46 (32.4%) were due to autoimmune diseases, 18 (12.7%) were due to tumors, 7 (4.9%) were due to other diseases, and in 20 (14.1%) the cause was still unknown after hospitalization. In conclusion, infection is the main cause of FUO. Autoimmune diseases and malignant tumors are both significant causes. Most patients with an FUO were ultimately diagnosed with various examinations and careful analysis.