Endometrosis is a chronic degenerative syndrome of the lining of the uterus in mares. There are no explicit symptoms and the cause is also unknown, but the severity of endometrosis increases in parallel with the age and number of pregnancies of the mare. Endometrosis is confirmed by histological examination of an endometrial biopsy, which shows degeneration of blood vessels in the endometrium, and fibrosis of the tissue, along with the development of endometrial cysts. The most common classification for endometrosis in mares was published by KENNY u. DOIG (1986) and modified by SCHOON et al. (1992). The different degrees of endometriosis are divided into stage I, IIA, IIB and III. These changes cause subfertility; in pregnant mares, the changes in the endometrium can cause the placenta to fail, leading to miscarriage of the foal. Foals which are delivered at full term may be underdeveloped (dysmature). No effective treatment is known. Since the causes of endometrosis are unsettled there is not known much about how to prevent it.