We present a case of a 12-year-old girl with severe cyanosis due to abnormal moderator band producing a hidden trabecular component of the right ventricle, mimicking isolated hypoplasia of the right ventricle. A marked hypoplasia was confirmed by echocardiography and catheterization. At operation an anomalously thickened moderator band obstructing the apical infundibulum was found. Repair consisted of a section of the moderator band and closure of the atrial septal defect. Postoperatively the right ventricle showed normal dimension and function.