A study has been made of 79 children from zero to seven years of age admitted to hospital with fever of unknown origin (F.U.O.) of more than two weeks duration. Children in whom fever was not clinically observed after one week of hospitalization are not included. In 50 cases (63.2%), it was possible to establish a definitive diagnosis within the first fornight of admission. The most frequent cause of fever was that of infection, found in 51 children (64.5%), tuberculosis and urinary infections predominating with ten cases each. In another ten children neoplastic disease was diagnosed (mostly leucosis), and there was colagenosis in seven cases (8.8%). In another seven children, the etiology was not established. Mortality rate was 7.5%. Clinical history and exploration were of main importance in the orientation of the diagnoses. The findings of this study suggest that in all children presenting F.U.O., apart from hospitalization of at least one week, a very thorough anamnesis and clinical exploration are most important in establishing the diagnosis, along with a more or less aggressive approach to the problem according to the findings.