Background: Anovulatory follicles are those that reach the correct size but then fail to ovulate. Two pathological forms of anovulatory follicle exist in horses: persistant anovulatory follicles and haemorrhagic anovulatory follicles.Aim of the article: This article will focus on the most common morphological form, which is the haemorrhagic anovulatory follicle; why it is important, what our knowledge was and what new knowledge we have developed in the past five to 10 years.