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 < 0.005). Eosinophilia was higher among immigrants from Central and Eastern Europe (17.2%), followed by Latin America (14.8%), and North Africa (13.4%). The majority of blood donors did not take any medication related to eosinophilia, and none had any disease associated with eosinophilia. Conclusions: Eosinophilia is a frequent diagnosis in asymptomatic donors and it is not associated with being native-born or immigrant; however, in the migrant group there were differences according to the area of origin. The presence of eosinophilia was not associated with drugs or other obvious pathologies. A future prospective study must be performed in order to clarify the causes of eosinophilia in donors. Resumen: Antecedentes: La eosinofilia es una alteración analítica frecuente en la práctica clínica. En la población general su prevalencia no está bien definida. Objetivo: Determinar la prevalencia de eosinofilia en donantes de sangre. Material y métodos: Estudio descriptivo, transversal. Se incluyeron donantes de sangre del Centro de Hemoterapia y Hemodonación de Castilla y León desde enero a diciembre de 2016. Resultados: De un total de 63.941 donantes, 8.616 (13,5%) presentaron eosinofilia; 3.839 (44,6%) tuvieron eosinofilia relativa, 4.590 (53,3%) tuvieron eosinofilia leve, 185 (2,1%) tuvieron eosinofilia moderada y 2 (0,02%) tuvieron eosinofilia grave. De los donantes con eosinofilia, 6.401 (74,3%) eran hombres. La media de edad (± DE) fue de 41,4 años (± 12,6); 8.299 (13,5) eran autóctonas y 317 (13) eran migrantes. No hubo diferencias significativas respecto a la eosinofilia entre inmigrantes y nativos (p = 0,47). Hay diferencias significativas en la eosinofilia entre los diferentes colectivos de inmigrantes (p < 0,005). La eosinofilia fue mayor entre los inmigrantes de Europa Central y del Este (17,2%), seguida de lo originarios de América Latina (14,8%) y África del Norte (13,4%). La mayoría de los donantes de sangre no tomaron ningún medicamento relacionado con la eosinofilia, ni presentaban enfermedad asociada. Conclusiones: La eosinofilia es un diagnóstico frecuente en donantes, no se asocia con ser autóctono o inmigrante. Entre inmigrantes hay diferencias según el origen. La presencia de eosinofilia no se asocia ni con fármacos ni con otras patologías. Se debería realizar en un futuro estudios prospectivos para aclarar las causas de la eosinofilia en donantes. Keywords: Spain, Eosinophilia, Prevalence, Blood donors, Palabras clave: España, Eosinofilia, Prevalencia, Donantes de sangre