Clinical records of 160 patients admitted with fever of unclear origin were reviewed. Of them, all cases with fever of obscure origin (FOO) were selected based on the following criteria: at least two weeks with fever, with an oral temperature of 37.5 degrees C or more during hospitalization and whose clinical history, physical examination, hemocytologic data, erithrosedimentation rate, urinalysis, febrile tests, glutamic-piruvic transaminase, chest and abdomen radiographies were not suggestive of any specific diagnosis. 32 cases of FOO were found, but only the 30 which had been studied previously in another hospital were considered for analysis. Of these 30 patients, 18 were men and 12 women, with a mean age of 36.3 (range 19-64). Infectious diseases caused 40 percent and neoplastic disease 27 percent of cases. The single most frequent cause was non-Hodgkin lymphoma in four cases, followed by tuberculosis and Hodgkin's disease in three patients each. In four cases the cause of fever was not identified. Eleven patients required exploratory laparotomy; in nine of them it was usefull for diagnosis. Our results show a high proportion of neoplastic diseases, probably related with patient's selection and with intrinsic diagnostic difficulty of these kind of diseases.