An electron microscope study of 2 cases of epithelial downgrowth after penetrating keratoplasty is presented. Several differences were observed between the normal epithelium and downgrowth epithelium at the ultrastructure level. At the advancing edge of the downgrowth a flattened and fibroblastic endothelium was still present. An interaction area was observed between these 2 tissues. The epithelial downgrowth usually has 5 to 6 layers of cells and either replaces damaged endothelium or destroys it as it advances. The attachment of this downgrowth tissue to the Descemet's membrane at the centre of the graft seemed looser than attachment of a normal epithelium to Bowman's membrane. Near the host-graft junction the downgrowth epithelium, with basement membrane, was tightly attached to a fibrous retrocorneal membrane. Clinical and histological findings suggest that the absence of endothelium assists in the advance of the epithelial downgrowth, but it is not an essential factor. The causes of this downgrowth are thought to be (1) a thin host cornea; (2) incomplete cutting and wound incarceration of Descemet's membrane; and (3) tight sutures with disruption of an old cataract wound.