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Rationale: Blood eosinophil counts are used to inform diagnosis/management of eosinophilic asthma. Objectives: Examine blood eosinophil variability and identify factors affecting eosinophil levels to inform clinical interpretation. Methods: Post hoc analysis to understand eosinophil variability using data from four randomized controlled asthma trials. We examined 1 ) influence of intrinsic/extrinsic factors (comorbidities, medication, and patient history) using baseline data ( n  = 2,612); 2 ) monthly variation using placebo-treated patient data ( n  = 713); 3 ) stability of eosinophil classification (<150, 150-299, and ⩾300 cells/μl) in placebo-treated patients with monthly measurements over a 1-year period ( n  = 751); and 4 ) impact of technical factors (laboratory-to-laboratory differences and time from collection to analysis). Results: Of intrinsic/extrinsic factors examined, nasal polyps increased eosinophil levels by 38%, whereas current smoking decreased levels by 23%. Substantial seasonal differences in eosinophil counts were observed, with differences of ∼20% between July and January. Eosinophil levels between 150 and 299 cells/μl were least stable, with 44% of patients remaining in the same classification for seven of 10 measurements versus 59% and 66% of patients in the <150 and ⩾300 cells/μl subgroups, respectively. Measurements at different laboratories showed high association (Spearman's correlation coefficient, R  = 0.89); however, eosinophil counts were reduced, with longer time from collection to analysis, and variability increased with increasing eosinophil counts. Conclusions: Several intrinsic, extrinsic, and technical factors may influence, and should be considered in, clinical interpretation of eosinophil counts. Additionally, a single measurement may not be sufficient when using eosinophil counts for diagnosis/management of eosinophilic asthma.