Microscopic study of a keratoplasty specimen in a case of posterior polymorphous dystrophy demonstrated via serial sections a multilaminar Descemet's membrane with focal fusiform nodular protrusions of connective tissue. The latter are thought to represent the polymorphous lesions of the posterior limiting layers of the cornea seen clinically. Ultrastructurally, fibroflast-like cells lined the posterior surface of the cornea. It is postulated that those reported cases of PPD in which epithelial-like transformation of the endothelial layer has been described may represent more advanced cases, in which corneal oedema is likely to be a prominant feature clinically. An evolving process of metaplasia rather than a static one might explain the variety of altered endothelial cells, fibroblasts, and epithelial cells which have been reported on the posterior cornea in the few PPD keratoplasty specimens studied to date.