Lutembacher's syndrome is a very rare form of congenital heart disease that affects one of the chambers of the heart (commonly the atria) as well as a valve (commonly the mitral valve). It is commonly known as both congenital atrial septal defect (ASD) and acquired mitral stenosis (MS). Congenital (at birth) atrial septal defect refers to a hole being in the septum or wall that separates the two atria; this condition is usually seen in fetuses and infants. Mitral stenosis refers to mitral valve leaflets (or valve flaps) sticking to each other making the opening for blood to pass from the atrium to the ventricles very small. With the valve being so small, blood has difficulty passing from the left atrium into the left ventricle. Septal defects that may occur with Lutembacher's syndrome include: Ostium primum atrial septal defect or ostium secundum which is more prevalent.