An infant with neonatal Marfan syndrome is described who presented with arachnodactyly, distinctive dysmorphic features and prolapse of both atrioventricular valves and dilatation of both the aortic and pulmonary root. She died in cardiac failure shortly after pacemaker implantation, due to dysrhythmia and severe mitral insufficiency. At autopsy, apart from myxomatous changes of the valves and dilated aortic and pulmonary roots, an aneurysm of the sinus of Valsalva of the pulmonary valve and abnormal myxomatous connective tissue surrounding the AV node were also found. Molecular genetic studies showed a point mutation in the fibrillin 1 gene that creates a new N-glycosylation site, which has been described once before.