Eosinophilia in neonates is identified when the blood concentration of eosinophils exceeds the upper reference range limit. To avoid the potential pitfall of laboratory or technician error, perhaps the definition should be two subsequent eosinophil counts above the upper reference limit. The 95th percentile for blood concentration of eosinophils increases slightly over the first month following birth. Initially a count ≥1,200/µL would exceed the upper range, and by about four weeks a count of above 1,500/µL would exceed the upper limit . This latter value is similar to that generally used to define eosinophilia in adults . Adults with persistent eosinophilia are well advised to have the situation evaluated, because an association has been seen between persistent eosinophilia and end-organ damage . Some adults with persistent eosinophilia have elevated blood Interkeukin-5 (IL-5) concentrations . Some with hypereosinophilic syndrome have an eosinophilic leukemia involving a translocation in the tyrosine kinase gene .