The aim of this study was to determine the causes of fever of unknown origin, to evaluate new diagnostic tests and to elucidate risk factors for chronic or life-threatening disorders. The medical records of 113 children who had undiagnosed fever for at least 3 weeks were reviewed. Infection (N= 41) was the most frequent cause of fever of unknown origin. Respiratory tract infections were the most common causes in infants and endocarditis and tuberculosis were more frequent in older children. Neo-plastic disorders (N= 11) occurred in children older than one year. Juvenile rheumatoid arthritis (N= 9) was the most common collagen-vascular disorder (N= 15). Miscellaneous disorders and factitious fever occurred in 21 and 4 cases, respectively. Twenty-two patients remained undiagnosed. History and physical examination led to a final diagnosis in 81% of cases. Abdominal ultrasonography was performed in 71 patients (61%) and was helpful for diagnosis in 15%. Children with life-threatening or chronic disorders (N= 58) were older than those with self-limiting conditions (N= 55;P= 0.017). Cardiovascular and articular signs and symptoms were more frequent in the former group (P= 0.01).