A 2-week-old newborn girl underwent successful surgery in our clinic for critical subaortic stenosis caused by accessory mitral valve tissue, which, because of excessive growth, protruded into the left ventricular outflow tract. The preoperative pressure gradient below the aortic valve was 80 mm Hg. The operation consisted of resection of the accessory tissue through a combined aortotomy and atriotomy approach without residual pressure gradient and mitral valve incompetence. This approach is recommended to ensure that accessory tissue is removed without damaging the mitral valve.