Adequate surgical relief of left ventricular outflow obstruction (LVOO) in transposition of the great arteries (TGA) with atrio-ventricular concordance and intact ventricular septum, may to a significant degree depend on the severity of the lesion and the anatomic location of the ventricular septum and the mitral valve. A new approach to resection of subpulmonary obstruction in TGA through the left atrium and the retracted mitral valve is described. In contrast to dynamic stenosis, transmitral resection of a fixed subpulmonary narrowing will usually lead to a significant decrease of LVOO in TGA, with good clinical results.