These studies were performed to investigate what genital diseases could be diagnosed by ultrasonography in the mares associated with infertility. Of 159 thoroughbred breeders(4 to 18 years old), 31 mares which had been considered as the mares with reproductive disorders were examined by ultrasonography during the breeding season. 1. Hematoma in ovarian disordersl endometritis and pyometra in uterine disorders could be diagnosed by ultrasonography. 2.The rate of infertility was 19.5% out of 159 breeders and the rate of uterine disordres was 71.0% among the whole reproductive disorders. 3.Of 31 mares considered as the mares with infertilityi 26 mares were diagnosed by ultrasonography denoting 83.9% of diagnosis rate. 4.The highest average age of mares among the mares with reproductive disorders was 11.7 years in the mares with uterine disorders. The average age of the mares diagnosed as concepted was 7.5 years, which was much lower than that of the mares with reproductive disorders.5.Crushing one of twin vesicles was confirmed by ultrasonography. In conclusions ultrasonography was feasible for diagnosis of the mares with reproductive problems.