Rationale: On Wright-stained sputum cytospins, eosinophil differential of >1.2% is considered abnormal and ≥3% clinically relevant. We hypothesized that failure to consider free eosinophil granules (FEG), and the re-emergence of eosinophilia (at ≥1.2 and ≥2.3%) underestimate the prevalence of the eosinophilic phenotype.Methods: This is a retrospective analysis of our Institutional Review Board-approved clinical sputum database. Of the 24,176 examinations, 17,693 had viable cell counts from 9570 patients (6604 on one occasion, 2967 with two or more) with various airway diseases. In all samples, FEGs were semi-quantified to estimate the prevalence of eosinophilia at <1.2 and <2.3%. In those patients with sputum examined on more than one occasion, subsequent results identified re-emergence of eosinophilia at ³1.2 and ³2.3%.Results: Of those with intact cell counts (n=15,278), 8562 (56.0%) and 9690 (63.4%) would have been classified as clinically non-relevant eosinophilia of <1.2% and <2.3% respectively. Among those with two or more intact cell counts, 1142 (38.5%) and 1083 (36.5%) of samples had re-emergence of eosinophilia (at 1.2 and 2.3%), either when a previous neutrophilia resolved (n= 218/19.1%, n=312/28.8%) or due to a reduction in the dose of corticosteroids (n=634/21.4%, n=666/22.4%) with no significant differences between groups (p=0.018). There was a significant difference between the proportions of eosinophilic versus non-eosinophilic at 1.2 and 2.3%, using the presence of FEG and the non-intact cell count (p<0.001).Conclusions: A total of 3180 (32.8%) samples identified as non-eosinophilic, had clinically relevant airway eosinophilia that would not have been identified if the phenotypic classification was limited to ≥2.3% intact eosinophils on one occasion. This underestimation is likely to be significantly more if other means of airway eosinophilic activity were included and if mast cell activity and lymphocyte numbers, (not routinely quantified by sputum cytometry), could be contributing to the under-appreciation of airway T2 inflammatory processes.