Univentricular heart is a rare congenital defect that consists, in nearly 80% of cases, of double inlet left ventricle, to which both atrioventricular valves connect. A 31-year-old male patient was diagnosed with single ventricle at the age of 9. He had not received any surgical intervention and was not on any medication. Functional capacity was assessed as New York Heart Association class III. Both atrioventricular valves were found to open into the morphological left ventricle, which was located on the right; the hypoplasic right ventricle was on the left side of the left ventricle, the posteriorly located aorta emerged from the right ventricle, and the anteriorly located pulmonary artery emerged from the left ventricle. There was significant subvalvular and valvular stenosis in the pulmonary valve site. The disease is complex and it is very rare that untreated patients reach adulthood.