Marfan syndrome is classified as a heritable disorder of connective tissue, and it has been assumed to be an inborn error of protein metabolism particularly in collagen or elastin. We examined 41 Marfan syndrome patients and found in the corneal endothelium guttata-formation, considerable cell pleomorphism, non-reflecting endothelial black spots and in some cases abnormal looking endothelial cells. The relation of pathological endothelium to lens subluxation, axial length and other corneal findings is discussed.