A seven-year-old Thoroughbred gelding presented to Cornell University Hospital for Animals with a history of acute onset of colic starting on the same day. The owners reported flank-watching and rolling, and the referring veterinarian had obtained nasogastric reflux and treated unsuccessfully with flunixin meglumine. Physical exam at presentation revealed hyperthermia, tachycardia, hyperemic mucous membranes with a toxic line, markedly decreased gut sounds, spontaneous nasogastric reflux through a nasogastric tube, and many distended loops of small intestine on rectal palpation. Initial diagnostics included blood work, abdominal ultrasound and peritoneal fluid analysis. These tests showed severe dehydration with electrolyte imbalances, neutropenia with a left shift, azotemia, distended small intestine and inflammatory peritoneal fluid. Altogether, these findings strongly suggested small intestinal ileus, likely due to a mechanical obstruction. Surgical management was recommended with a guarded prognosis but the owner elected euthanasia. On necropsy a mesenteric rent resulting in jejunal strangulation was diagnosed. This report will include a description of the clinical case as well as overviews of mesenteric rents and endotoxemia in horses.