Background Fever is the most common sign in the paediatric emergency room. In small children, the absence of a detectable origin, after close clinical assessment, represents a diagnostic challenge, in the face of the risk of a serious underlying infection.

Aims To describe frequent causes and agents of acute fever without source (FWS), in newborns and infants until 36 months of age. To evaluate the requested examinations utility in diagnosis and severity prediction.

Patients and Methods Retrospective study based on the 2007 inpatient clinical records. Inclusion criteria: children ⩽36 months admitted from the emergency room with FWS (rectal temperature >38°C). Exclusion criteria: patient on antibiotics. Analysed variables: age, gender, epidemiological context, vaccination status, clinical manifestations, diagnostic exams, treatment and outcome. χ² Test and Student’s t test were used for statistical analysis (α  =  0.05).

Results 115 children were admitted: three newborn, 37 with 29–90 days and 75 with 4–36 months. Most common causes were: urinary tract infection (53%), probable viral infection (23.5%), occult bacteraemia (12.2%) and meningitis (5.2%). Escherichia coli and Streptococcus pneumoniae were the more frequently isolated agents in urine and blood cultures, respectively. Toxic appearance, temperature >39.5°C, white blood cell count >15 000/mm³ and C-reactive protein >40 mg/l had a positive predictive value for serious underlying infection. 76.5% of the patients received empiric antibiotic therapy.

Conclusions Urine analysis and culture seem to be the most useful examinations when evaluating FWS. The progressive introduction of new vaccines is continuously changing epidemiology and therefore the most cost-effective strategy approach to FWS.

Keywords: children; fever without source; infant; newborn