Abstract. This report describes 2 cases of spontaneous aortic dissecting hematoma in young Border Collie and Border Collie crossbred dogs. Histology was performed in one of the cases involving an unusual splitting of the elastin present within the wall of the aorta, consistent with elastin dysplasia as described in Marfan syndrome in humans. The first case involved a young purebred Border Collie that died suddenly and the second case involved a Border Collie crossbred dog that died after a 1-month history of seizures. Gross lesions included pericardial tamponade with dissection of the ascending aorta in the former case and thoracic cavity hemorrhage, mediastinal hematoma, and aortic dissection in the latter. Histologic lesions in the case of the Border Collie crossbred dog included a dissecting hematoma of the ascending aorta with elastin dysplasia and right axillary arterial intimal proliferation.