A 14-year-old Thoroughbred broodmare was evaluated for signs of mild to moderate abdominal pain of 5 hours' duration. Exploratory celiotomy revealed a 20 x 8-cm subserosal hematoma associated with a focal serosal perforation overlying a muscular tear along the parietal surface of the stomach. The odor of intestinal tract gas was detected, but gross contamination of the abdomen was not evident. Concurrent partial volvulus of the small intestine was evident. Removal of the large colon from the abdomen appeared to coincide with propagation of the serosal perforation to a full-thickness gastric rent. The tear was repaired by use of a 2-layer inverting closure. The horse had evidence of an acute gastrointestinal tract hemorrhage 10 days after surgery, but responded to treatment. At a 2-year follow-up evaluation, the mare had foaled once and had not experienced recurrent signs of abdominal disease.