Due to its low prevalence and because there is lack of awareness about it, Loeys-Dietz syndrome is often misdiagnosed as Marfan syndrome, which has similar skeletal abnormalities and aortic pathology. However, the differential diagnosis between these two connective tissue diseases is critical because they correspond to different surgical indications and surgical decision-making. We report two cases of successful thoracoabdominal aortic replacement in patients with previously undiagnosed Loeys-Dietz syndrome.