Introduction: We examined the causative diseases of patients presenting with a chief complaint of fever.Methods: The patients were new patients visiting the Department of General Medicine at the University of Tsukuba Hospital between April 2018 and March 2020. The chief complaints were obtained from the patients' medical records, categorized according to the International Classification of Primary Care Second Edition. Patients with fever (A03) as the chief complaint were enrolled and their diagnoses were investigated.Results: Of 610 eligible patients, 85 had fever as the chief complaint. Causative diseases were infectious disease in 31 patients (36%), noninfectious inflammatory diseases in nine patients (11%), malignancy in three patients (3%), others in 14 patients (16%), unknown in 19 patients (22%), and exhibiting no pathologic conditions in nine patients (11%). Specific diagnoses were viral infection for the infectious disease patients, collagen diseases for the noninfectious inflammatory disease patients, hematologic diseases for the malignancy patients, and drug-related issues for others.Conclusion: The most common cause of fever was infectious diseases. Some patients were concerned about disease even when their temperatures were in the physiologic range.