A 18-year-old Dutch Warmblood mare was referred for colic. Upon arrival, lethargy, blindness, head pressing, ataxia, and circling were the main clinical signs. On rectal examination a hard mass and oedema around the cranial mesenteric artery were palpated. Plasma liver enzyme activities and the ammonia level were elevated. Atrial fibrillation with a pulse frequency of 36-52 beats per minute was noticed. On both sides a holosystolic murmer with the maximum intensity on the right side could be auscultated. Postmortem examination revealed eccentric hypertrophy of the right atrium and a pale spotted myocardium, most prominently of the right ventricle, with secondary venous congestion of the azygos and mesenteric veins. The liver changes were indicative of chronic congestion. Despite the normal pulse rate, it appeared that congestive heart failure due to cardiomyopathy, was responsible for the presenting symptoms of this patient.