The aim of this study was to detect pathological changes in the endometrium of mares with the endometrial biopsy technique and to classify these changes according to distribution and severity, and finally to investigate the relation of these changes to age and foaling rates. Biopsies were obtained from 53 mares which had not foaled for at least two years who were at different stages of the estrous cycles. Pathological endometrial changes were observed in 92.3 percent of mares with the majority having fibrosis (73.6%) and chronic infiltrative changes (50.9%). All categories included mares of different ages. Four mares were classified as category-I, 7 as category IIa, 10 as category-IIb and 32 as category-III. The foaling rate obtained for categories-I, IIa, IIb and III were 75, 42.8, 10 and 0% respectively for the breeding season following the collection of endometrial biopsy. It is apparent from the data that there is a meaningful association between histological categories and foaling rate indicating that endometrial biopsy should be used as a diagnostic technique in the evaluation of fertility potential of mares.