Background: Asthma is a heterogeneous disease in which the interaction of genetic and environmental factors plays a major role. The significance of blood eosinophil is unclear. The aim of the study was to determine the significance of blood eosinophil count in moderate to severe asthmatic children of preschool and school age. Methods: This was a prospective cross-sectional study performed from 2011 to 2015 including children from the Severe Asthma Molecular Phenotype (SAMP) cohort at Trousseau Hospital (Paris, France). We included children with severe and moderate asthma, or severe and moderate recurrent wheeze, aged from 1 to 15 years at the time of exploration. Results: We analyzed data from 402 children: 248 of preschool age and 154 of school age. Blood eosinophil count third quartile thresholds were 322 and 600 cells/μL for the preschool- and school-age groups, respectively. In multivariate analysis, a blood eosinophil count over this threshold was associated with elevated total IgE (OR=5.33; P<0.01), multiple hospitalizations for asthma attacks (OR=4.96; P=0.03), and a maternal history of asthma (OR=4.91; P=0.01) in preschool children; and with staphylococcal toxin-specific IgE (OR=2.75; P=0.03) in children of school age. Random forest analysis reinforced these results. Conclusion: High blood eosinophil count is linked to both atopic features and control of asthma with different parameters associated with these features depending on age.