A retrospective study analyzed 215 patients with fever of unknown origin (FUO) diagnosed between 2009 and 2010. The most common causes were infectious diseases (42.3%), connective tissue diseases (32.1%), miscellaneous (10.7%), and neoplasm (6.5%). The remaining 18 cases (8.4%) remained undiagnosed. The most common specific causes were tuberculosis (17.6%), adult-onset Still disease (53.6%), and non-Hodgkin lymphoma (42.9%). Fever patterns varied, with regularly intermittent fever associated with urinary infections and irregularly intermittent fever linked to infective endocarditis. The study found that CTDs were more common in women, while IDs and neoplasm were more common in men. Patients with neoplasm had splenomegaly and low hemoglobin levels, while those with CTDs had higher white blood cell counts and ESR. The study also noted that the frequency of IDs and miscellaneous causes decreased, while neoplasm and undiagnosed cases increased over time. These changes may be attributed to advances in diagnostic techniques. Despite these changes, IDs remain the most common cause of FUO in northern China. The study highlights the importance of identifying specific fever patterns to improve the diagnosis of FUO.A retrospective study analyzed 215 patients with fever of unknown origin (FUO) diagnosed between 2009 and 2010. The most common causes were infectious diseases (42.3%), connective tissue diseases (32.1%), miscellaneous (10.7%), and neoplasm (6.5%). The remaining 18 cases (8.4%) remained undiagnosed. The most common specific causes were tuberculosis (17.6%), adult-onset Still disease (53.6%), and non-Hodgkin lymphoma (42.9%). Fever patterns varied, with regularly intermittent fever associated with urinary infections and irregularly intermittent fever linked to infective endocarditis. The study found that CTDs were more common in women, while IDs and neoplasm were more common in men. Patients with neoplasm had splenomegaly and low hemoglobin levels, while those with CTDs had higher white blood cell counts and ESR. The study also noted that the frequency of IDs and miscellaneous causes decreased, while neoplasm and undiagnosed cases increased over time. These changes may be attributed to advances in diagnostic techniques. Despite these changes, IDs remain the most common cause of FUO in northern China. The study highlights the importance of identifying specific fever patterns to improve the diagnosis of FUO.