Of 222 infertile Arabian mares, unilateral ovarian enlargement was detected in three cases. Ultrasonography suggested the presence of ovarian tumours. Blood samples were taken for hormonal analysis, and the mares underwent surgery for ovarian exploration. Ovarian specimens were processed for microscopic examination. On histopathology, an adult-type granulosa cell tumour (GCT), a mixed germ cell tumour and a lowgrade Sertoli cell tumour were diagnosed. The GCT (6.5 cm in diameter) was rounded and cystic, with a central hypoechogenic area. The ovarian tissue was entirely replaced by polygonal neoplastic cells. The mixed germ cell tumour (15.3 cm in diameter) was hard and non-homogeneous. The ovary contained insular patterns of round, oval or spindle cells with myxomatous and hyalinised stroma. The stroma also contained osteoid tissue. The ovarian low-grade Sertoli cell tumour (19×20 cm) was a solid, fairly round homogeneously echogenic structure. The ovarian tissue was completely replaced by aggregated tubules lined by Sertoli-type cells surrounded by abundant fibrous stroma. The three types of tumours secreted relatively low levels of estradiol 17β and progesterone, whereas the level of inhibin-B was extremely high in the case of the Sertoli cell tumour. In conclusion, this study described three types of ovarian tumours causing infertility in Arabian mares. Furthermore, the Sertoli cell tumour is described here for the first time in mares.