Endometritis is a major cause of infertility in mares. The aim of this study is to evaluate the preovulatory follicle (POF) vascularization (A mix, A red, A blue), POF diameter, POF wall thickness, and uterine diameters in mares with or without endometritis. Ten healthy mares and 10 mares with endometritis diagnosed by the combination of transrectal palpation, ultrasonographic examination, and cytology brush were enrolled in the study. Data of the groups obtained at 2 days before the ovulation (day -2) were compared with Student's t-test. Correlations of the parameters were determined by the Pearson correlation test. To the best of our knowledge, this is the first report indicating a lower vascularization of A mix, A red, A blue, and a significantly lower wall thickness in the POFs of the mares with endometritis compared to the healthy ones (p < 0.01, p < 0.05, p = 0.06, and p < 0.001, respectively). Also, a striking and novel inverse correlation between POF wall thickness and uterine diameter (r = -0.785, p < 0.001) and moderate correlations between POF wall thickness and POF vascularizations of A mix and A red (r = 0.436, p = 0.055, and r = 0.427, p = 0.060, respectively) were determined. (C) 2020 Elsevier Inc. All rights reserved.