A 1-week-old Morgan filly was evaluated because of acute signs of abdominal pain of 7 hours' duration. On admission, physical examination findings were unremarkable; however, radiography of the abdomen revealed slight distention of the small intestine. Signs of abdominal discomfort were detected during several hours of observation. Abnormalities were not evident during gastroscopic evaluation. Therefore, exploratory laparotomy was performed. The only abnormal finding was infarction of the left ovary secondary to a 720 degrees torsion of the mesovarium. The vascular pedicle was ligated and the ovary was removed. Recovery from anesthesia and surgery was satisfactory, and the foal was discharged from the hospital 5 days after surgery. Two years later, it appeared to be clinically normal. Review of the recent veterinary literature failed to find reports of ovarian torsion as a cause of signs of abdominal pain in horses. Ovarian torsion should be considered as a differential diagnosis in fillies with acute signs of abdominal pain, especially when laparotomy fails to reveal abnormalities associated with the gastrointestinal tract.