Optical, scanning and transmission electron microscopy examinations were performed in two cases of polymorphic posterior corneal dystrophy. The first case demonstrated typical clinical features, histologic examination revealing the presence of granular deposits in the posterior stroma, and an anarchic fibrillary zone between the stroma and Descemet's membrane. The latter was thickened, with a normal striated fetal zone, an intermediate zone composed of fibrils and long and short collagen fibers, and a normal adult zone. Cells lining the anterior chamber were of the typical endothelial type. Clinical signs in the second case were less evocative even though the father of the patient presented a typical form. Histology showed similar granular deposits in the posterior stroma, and the fibrillary zone beneath Descemet's membrane, which was thickened and delaminated and composed of three layers. Anterior chamber lining cells were disposed in several layers and had numerous surface microvillosities. They were linked by many desmosomes. These cells were therefore of the epithelial type. Comparing the findings in these two cases demonstrates that whereas many common signs existed there was a fundamental difference between the cell types on the posterior surface of the cornea. A review of all possible hypotheses led to the conclusion that the epithelial type cells are of mesenchymatous origin, similar to endothelial cells, as they possess the identical property of secreting a normal adult type of Descemet's membrane and of ensuring deturgescence of the corneal stroma. The pathological mechanisms involved, probably biochemical in nature, which provoke this anomaly can be fairly precisely determined as developing during fetal life, but they remain totally inexplicable. This hypothesis of a mesenchymatous cell developing into an epithelial type allows polymorphic posterior dystrophy to be regarded as a distinct entity, with similar features to other neonatal posterior dystrophies and a vast group of endothelial dystrophies including, among others, congenital edematous dystrophy.