Fever of unknown origin has become a high cost entity that requires availability of several diagnostic tests. Objective: to describe the clinical and etiological characteristics of patients with fever of unknown origin. Materials and methods: a descriptive study of a series of cases, retrospective, of inpatients in the internal medicine service for fever of unknown origin, between June 1 of 2009 and March 31 of 2012, in the Hospitals of San José and the Infantile University of San José. Results: total population 36 patients, average age 42.6 years (SD 19.4), 58.3% men, 25% university students, 66% diagnosed in the Hospital of San José. The most frequent etiologies were: infectious diseases (30.5%), collagenosis (19.4%), neoplastic diseases (11.1%), miscellaneous (2.7%), the cause was not found in 36%. Conclusion: in our study, the patients were young with little medical history. The infectious pathology was the most frequent, followed by inflammatory diseases and the etiological diagnosis was impossible in 36%. In some cases, tests that are considered useful were not requested. It is necessary to take into account this information to improve the diagnostic approach of these patients. Abbreviations: FOD, fever of unknown origin.