Background: Although eosinophilia is a condition that appears frequently in clinical practice, its prevalence in the general population in Spain is not well-defined. Objective: To determine the prevalence of eosinophilia in blood donors. Methodology: Descriptive, cross-sectional study. Asymptomatic blood donors from the Haemotherapy and Blood Donation Centre of Castilla-León during January 2016 to December 2016 were included in the analysis. Principal findings: Of a total of 63,941 donors, 8616 (13.5%) presented with eosinophilia, 3839 (44.6%) had relative eosinophilia, 4590 (53.3%) had mild eosinophilia, 185 (2.1%) had moderate eosinophilia and 2 (.02%) had severe eosinophilia. Of the donors with eosinophilia, 6401 (74.3%) were men. The mean of age (±SD) of was 41.4 years (±12.6); 8299 (13.5) were autochthonous, and 317 (13.0) were migrants. There were no significant differences in the presence of eosinophilia between immigrants and natives (p = 0.47). Eosinophilia in the migrant cohort showed significant differences (p