Unexplained fever is a common clinical dilemma in internal medicine and a frequently seen disease in rheumatology. The concept was first proposed by American physicians and refers to fever lasting more than 3 weeks (including 1 week of hospitalization), with a temperature >= 38.3 degrees, which is not clearly diagnosed after a complete history, physical examination and routine laboratory tests. It was found that 1/3 of all febrile patients were associated with infectious diseases (mostly tuberculosis, especially extrapulmonary tuberculosis); 1/3 with neoplastic diseases; and 1/4 with autoimmune diseases, including systemic lupus erythematosus, vasculitis, dry syndrome, rheumatoid arthritis, adult Still’s disease, and other connective tissue diseases.