Equine endometritis eosinophilica (EE) is rarely described and its diagnostic criteria are not defined. The aim of this study was to characterize histological features of EE. A data base (1995-2013) was searched for biopsies with increased eosinophils. This study included all biopsies with this diagnosis and representative biopsies without this record. The definition of equine EE was based on criteria for EE in women and the results of the determination of physiological numbers of eosinophils within the equine endometrium. EE was diagnosed in 55 mares. Biopsies of 10 mares contained eosinophils exceeding the physiological range, but no EE; the diagnosis of eosinophilic infiltrates (EI) was applied. Those of the remaining mares (n = 126) displayed eosinophils within the physiological range (EWPR). An irregular glandular differentiation during the breeding season was detected in 25% of mares with EE, 33% of mares with EI and 24% of the mares with EWPR. Most mares with EE (93%), EI (90%) and EWPR (72%) showed endometrosis; it was high grade in 11% with EE and 7% with EWPR. Endometritis was diagnosed within 56% of mares with EE, 40% of mares with EI and 37% of those with EWPR. In mares with EE suppurative endometritis dominated (58%) and in those with EWPR non-suppurative endometritis (58%). This study indicates EE as a primary fertility reducing disease. Results suggest an association between eosinophilic infiltration and the presence of neutrophils. Further, they provide the basis for future studies into the pathogenesis, prognosis and therapy of EE.