This report describes the use of ultrasound to diagnose right dorsal displacement of the large colon (RDDLC) in 13 horses prior to surgery. Horses had ultrasonographic examinations performed of the right lateroventral aspect of the abdomen upon admission to the hospital with a 2–5 MHz broadband curvilinear sector scanning transducer after alcohol was used to wet the hair. First, the caecal vessels were identified in the right flank and followed medially and cranially. Next, each intercostal space, from caudal to cranial, was scanned from dorsal to ventral evaluating for abnormally-located mesenteric vessels associated with the large colon. Abnormally-located mesenteric vessels associated with the large colon, distinct from the caecal vessels, were identified in 13 of 23 horses with a diagnosis of RDDLC obtained at exploratory laparotomy. In horses, ultrasonographic visualisation of mesenteric vessels along the right lateral abdomen, dorsal to the costochondral junction in at least 2 intercostal spaces, distinct from the caecal vessels, is consistent with a surgical diagnosis of RDDLC.