Fuchs endothelial corneal dystrophy is the most common genetically determined degenerative disease of the cornea. In Polish patients the dystrophy is a leading indication for lamellar posterior keratoplasty. The genetic background of Fuchs endothelial corneal dystrophy is complex and heterogeneous. A number of TCF4 gene variants have been strongly associated with the development of this disorder with the most important of them being the trinucleotide repeat expansion CTG18.1. The aim of the study is to present this novel and extraordinarily strong genetic association with Fuchs endothelial corneal dystrophy. Studies on the impact of CTG18.1 on corneal endothelial cells may help to explain the molecular mechanism involved in the pathogenesis of the corneal dystrophy. This could significantly improve diagnostics and therapy of Fuchs endothelial corneal dystrophy patients.