In this survey involving 10 hospitals, we analyzed data on 80 Japanese patients from the Shin'etsu area (Nagano-ken and Niigata-ken) who were observed for fever of unknown origin (FUO). Our objectives were to identify the underlying causes and the relevant diagnostic methods. Fourteen of the patients died of the underlying illness. The cause of the FUO was infection in 43 patients, allergic or autoimmune disease in 13, neoplasm in 7, miscellaneous causes in 3, and undetermined in 14. FUO was self-limited in 13 patients and persistent in one patient. Methods successfully used to establish the final diagnosis in 66 patients were: evaluation of the clinical course or response to treatment in 16, serologic tests in 12, bacteriologic studies in 10, biopsy in 9, cytologic examination in 6, conventional radiology in 6, necropsy in 3, endoscopy in 2, and biochemical testing in 2.