In patients with transposition of the great arteries (TGA), both the D and L forms, an aneurysm of the membranous ventricular septum (AMS) produces subpulmonic stenosis due to the higher right ventricular pressure which forces the aneurysm to protrude into the left ventricular, i.e., subpulmonic, outflow tract. The clinical signs and symptoms, hymodynamic findings as well as surgical results were analyzed in eight patients with TGA and AMS. The presence of an AMS should be suspected from hemodynamic data consisting of a combination of elevated left ventricular pressure, gradient across the left ventricular outflow tract and presence of a small ventricular septal defect with or without pulmonary artery hypertension. The AMS can be demonstrated by a right ventricular injection in the lateral view. The anomaly needs to be corrected at the time of the Mustard procedure. If uncorrected it may lead to postoperative death or progressive obstruction. In the presence of an aneurysm even small ventricular septal defects should be closed by a patch and the aneurysm should be excised.