- Fever of unknown origin (FUO) is still a diagnostic challenge for the family physician and the internist. In the last decades a few reports have emphasized a changing pattern in the prevalence of the 3 main categories of FUO: infections, malignancies and collagen diseases. a. to find out if the changing pattern among the main diagnoses in patients with FUO is comparable to previous reports. Medical files of patients that were admitted in two 450 beds rural Israeli hospitals were checked by two physicians. All files of patients with either the diagnosis of FUO, or files of patients with fever that were hospitalized for a week or longer in internal medicine departments were reviewed. 101 files of patients fulfilling the criteria of FUO were found. Surprisingly 54.5% of them had infectious diseases, 7.9% had malignant disease and only 2% had collagen disease. a. Infectious diseases are still the leading cause of FUO among Israeli patients. b. The prevalence of infectious and malignant diseases is comparable to other studies, however the low rate of connective tissue diseases in our study is unusual. c. The rate of undiagnosed FUO was remarkably high (32.7%), although all these patients recovered during hospitalization and probably had self limited infectious (viral) disease.