A 20-year-old Thoroughbred mare was evaluated because of a 2-year history of infertility. The mare had normal estrous cycles and had been bred 7 times by different stallions. Ultrasonographic examination revealed a homogeneous hyperechoic intramural mass in the tip of the right uterine horn; the mass was also detected via hysteroscopy Unilateral ovariectomy and partial hysterectomy were performed by use of a hand-assisted laparoscopic technique. Leiomyoma was diagnosed via histologic examination of the mass. Unilateral ovariectomy and partial hysterectomy are recommended in mares with leiomyoma in a uterine horn, especially if the tumor is associated with infertility. The hand-assisted laparoscopic technique allows direct visualization of abdominal structures and accurate placement of ligatures without applying tension on the broad ligament, and eliminates the risks and costs of general anesthesia.