Endometrosis is a chronic degenerative syndrome of the lining of the uterus (the endometrium) in mares. The cause is 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. 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.
The etymology of endometrosis is from the Greek "endos" (inside), "metra" (womb) and "-osis" (disease). This term was adopted in 1992; prior to that, endometrosis was variously known as chronic degenerative endometritis, endometrial fibrosis, or chronic endometrial disease.