SummaryThe objective of this study was to examine the relationship between pregnancy results and the presence of inflammation detected by PMNs (cytology or histology) and microbial growth from an endometrial biopsy. Following recovery of the endometrial biopsy a quick stained cytology slide and bacterial culture was performed. After this was done the biopsy was fixed in 4% formaldehyde. Histology was evaluated for presence of PMNs and a Kenney score was assigned to each mare using the guidelines proposed by Kenney and Doig (1986). Significant correlation was detected between bacterial growth and PMNs detected by histology (p=0.0082) and PMNs detected by histology and day 70 pregnancy rate (p= 0.0022), but no significant correlation could be determined between cytology or bacterial growth and day 70 pregnancy rate or cytology and presence of PMNs detected by histology. In a logistic regression model following factors were evaluated to determine impact on pregnancy rate at day 70: 1) Histological detection of PMNs, 2) Cytology, 3) Bacteriological culture, 4) Mare age and 5) Kenney score. The only significant factor with impact on day 70 pregnancy rate was histological detection of PMNs (p=0.003). Significant correlation between Kenney groups and pregnancy rates was found in both the 75 initially included mares and in 74 mares from the same two stud farms that were retrospectively included in the study (p=0.0069 and p <0.0001), respectively. The study suggests the use of histological detection of presence of PMNs to diagnose endometritis, due to the statistical association and strong negative impact of PMNs present within the endometrium on chances of establishing a pregnancy.Keywords: Reproduction, endometritis, cytology, microbiology, histology.