Summary: During routine examination of equine endometrial biopsies periglandular accentuated mononuclear inflammatory cell infiltrates (PAMC) were often noted. The objective of this study was to characterise PAMC morphologically and immunohistologically with regard to their role in the mare's endometrium and their potential impact on the development of endometrosis. For this purpose, haemalaun and eosin (H&E) stained sections of endometrial biopsies from 754 mares at the Institute of Pathology (Faculty of Veterinary Medicine, University of Leipzig) during 2009 were evaluated retrospectively for the presence of PAMC. They were diagnosed in 133 biopsies which were subsequently assessed in detail. Of these, 72 randomised samples were examined using special staining and immunohistochemical techniques to analyse the inflammatory cell populations involved in PAMC (CD3: T-lymphocytes; CD79A: B-lymphocytes; MAC387: macrophages; methyl green-pyronin staining: plasma cells), and characterise the affected gland epithelium and stromal cells by detecting intermediate filaments and microfilaments (vimentin, desmin, a-smooth-muscle-actin) and assessing the integrity of the basal lamina of affected glands by detection of the basal lamina component laminin. PAMC occurred in about 18 % of all biopsy samples investigated during 2009 (133/754) and 96 % of these showed concurrent endometritis and/or endometrosis. The endometritis was composed of inflammatory cells similar to those observed in PAMC (71%) or it contained varying numbers of neutrophils (28 %). Whereas mild-to-moderate endometrosis could be observed in the endometrium of 95% of the cases, the majority (71%) of the endometrial glands surrounded by PAMC did not show any fibrotic alterations. In many PAMClocalisations (48 %) an infiltration of inflammatory cells into the glandular epithelium could also be seen. PAMC consisted mainly of T-lymphocytes and plasma cells. In addition, smaller numbers of B-lymphocytes and macrophages could also be detected. Within all PAMC foci, structural alterations were apparent in the basal lamina. Vimentin (60%), desmin (17%) and a-smooth-muscle-actin (a-SMA; 28 %) were expressed by stromal cells surrounding the lesion. PAMC cannot be considered solely as a histopathological feature of lymphoplasmacell endometritis, but it is reasonable to interpret PAMC as a regular component of mucosa associated lymphoid tissue (MALT) in the equine endometrium. Furthermore the expression patterns of vimentin, desmin and a-SMA in affected epithelial and stromal cells are comparable to findings in early stages of equine endometrosis as is the myofibroblast differentiation of perilesional stromal cells. PAMC were closely associated with alterations of the basal lamina of the endometrial glands involved. In general, the lesions involved in the basal lamina play a central role in the pathogenesis of equine endometrosis. Therefore, PAMC might be interpreted as one of the triggering factors for the development of endometrosis.Keywords: endometrosis / endometriti...