A 16-year-old Friesian gelding was admitted to the Faculty of Veterinary Medicine (GhentUniversity) with complaints of exercise intolerance, pale mucous membranes, tachycardia and anirregular heart rate. General examination revealed decreased heart sounds and bronchovesicularsounds in the ventral lung fields, combined with a low hematocrit, which led to the suspected diagnosisof hemothorax. The diagnosis of hemothorax was confirmed by ultrasonographic examination of thethoracic cavity and thoracocentesis. Because it was a Friesian horse, aorto-pulmonary fistulation wassuspected, but this was excluded by thorough echocardiographic examination. The electrocardiogramshowed that the irregular heart rate was caused by atrial premature contractions. The horse washospitalized and was treated conservatively with broad spectrum antibiotics. Because both thehematocrit and the ultrasonographic images evolved well, the horse could leave the clinic after 15days. Re-examination after six weeks showed no more abnormalities.