OBJECTIVE: The aim of the study was to evaluate the impact of procalcitonin (PCT) measurement on antibiotic use in children with fever without source. METHOD: Children aged 1 to 36 months presenting to a pediatric emergency department (ED) with fever and no identified source of infection were eligible to be included in a randomized controlled trial. Patients were randomly assigned to 1 of 2 groups as follows: PCT+ (result revealed to the attending physician) and PCT- (result not revealed). Patients from both groups also had complete blood count, blood culture, urine analysis, and culture performed. Chest radiography or lumbar puncture could be performed if required. RESULTS: Of the 384 children enrolled and equally randomized into the PCT+ and PCT- groups, 62 (16%) were diagnosed with a serious bacterial infection (urinary tract infection, pneumonia, occult bacteremia, or bacterial meningitis) by primary ED investigation. Ten were also found to be neutropenic (<500 x 10(6)/L). Of the remaining undiagnosed patients, 14 (9%) of 158 received antibiotics in the PCT+ group vs 16 (10%) of 154 in the PCT- group (Delta -2%; 95% confidence interval [CI], -8 to 5). A strategy to treat all patients with PCT of 0.5 ng/mL or greater with prophylactic antibiotic in this group of patients would have resulted in an increase in antibiotic use by 24% (95% CI, 15-33). CONCLUSION: Semiquantitative PCT measurement had no impact on antibiotic use in children aged 1 to 36 months who presented with fever without source. However, a strategy to use prophylactic antibiotics in all patients with abnormal PCT results would have resulted in an increase use of antibiotics.