Atresia of the mitral valve, often reported with cases of single ventricle or the hypoplastic left heart syndrome, is an uncommon cardiac lesion. Maude Abbott1 in her classic monograph described 5 specimens encountered in 1,000 postmortems of patients with congenital heart disease. It has been reported by Keith, Rowe, and Vlad2 that it occurs in 2% of their cases of congenital heart disease, or once in 50,000 children. Teller3 referred to 88 cases he was able to collect from the literature. The etiology of this malformation is not known, although several theories as to pathogenicity have been advanced. Among these are theories that aberrations in the usual bending of the primary cardiac tube (Spitzer4,5,6) occurs; that there is maldirected g r o w t h of the interventricular septum (Moenckeberg7); that fusion between the septum primum and the endocardial cushions (Brockman8) takes place; that