Background: Extreme leukocytosis, defined as a peripheral white blood cell count >25,000/mm3, may alarm clinicians and prompt extensive evaluation in infants with fever, especially in the pediatric emergency department. Methods: We reviewed data from infants aged 3-36 months with extreme leukocytosis, fever and the risk of severe bacterial infections (SBI) at our institution from July 2010 to December 2012 - a period after the universal introduction of pneumococcal vaccine. Results: SBI was recorded in 57 (39%) of the 147 infants. The most common SBI were segmental or lobar pneumonia, in 28 (19%) patients, and urinary tract infection in 16 (10.9%) patients. Three patients had positive blood cultures. Conclusion: All well-looking febrile infants with WBC >25,000/mm3 should undergo a chest radiograph unless there are clear physical findings that indicate a different etiology. Urine culture should be considered in females.
