The case of a 11 year-old mix breed dog is being described. It initially presented for watery diarrhea and inappetence. During the diagnostic workup, the dog developed a sustained ventricular tachycardia. Echocardiography showed a mass extending from the wall of the right ventricle which filled up the ventricle almost completely. Histopathologic examination of the mass after euthanasia of the dog revealed a hemangiosarcoma.