Corneal discs from 10 cases of posterior polymorphous dystrophy (PPD) and 20 cases of congenital hereditary endothelial dystrophy (CHED) were compared and contrasted using light and electron microscopy. Secondary epithelial changes were similar in both diseases but spheroidal degeneration of stroma was seen more commonly in dominant CHED and not at all in PPD, when band, calcific, keratopathy was commoner. Changes at the level of Descemet's membrane showing failure to regulate growth were seen in recessive CHED whereas dominant CHED and PPD were both associated with development of a fibrillary posterior collagen layer (PCL). Grotesquely banded PCL was also seen in some cases of PPD. Endothelial changes included vacuolation, development of microvilli and desmosomes in both diseases but multilayering was more common in PPD. The viscous layer of the cornea was seen by TEM in one case of PPD.