Background: Atrioventricular septal defects (AVSD) are caused by a lack of development in atrioventricular endocardial cushions. Its spectrum varies from partial to transitional, or intermediate forms with a common AV valve and 2 orifices, or even the complete form. Aim: The aim of this study is to present a case of a woman in the fifth decade of life with Down syndrome and a transitional AVSD, diagnosed with echocardiography. Case presentation: This is a patient in the fifth decade of life with Down syndrome, transitional AVSD, and severe pulmonary hypertension. Her vital signs were normal, oxygen saturation of 89% on room air. Auscultation revealed pronounced pulmonary component of the second heart sound, diastolic murmur in second left intercostal space, and moderate holosystolic heart murmur that radiated to axilla. The diagnosis was made with a transthoracic echocardiogram. The patient is receiving only medical treatment and since her last consult she has New York Heart Association (NYHA) functional class II. Conclusion: This case demonstrates the natural history of a patient in her fifth decade of life with down syndrome and complex congenital heart disease, despite having received only medical treatment, she is in functional class II. Echocardiography is the noninvasive technique of choice in the diagnosis and follow-up of patients with congenital heart diseases, as seen in this case.