We report an unusual case of posterior polymorphous dystrophy in which corneal failure began within a few weeks of birth. Histopathologic findings included the presence of abnormal corneal endothelial cells with many microvilli on the surface. Descemet membrane was severely attenuated, and there was a thick posterior collagenous layer consisting of numerous fibroblastlike cells in a fibrillar extracellular matrix; ultrastructural immunocytochemistry showed this to contain tenascin, fibronectin, and collagen type I. Few histopathologic data on this disease at such an early age have been available, and to our knowledge, the composition of Descemet membrane has not been examined before. The microvilli-covered cells are shown to be present from the outset of the disease, not just in long-standing cases as in previous reports; changes in Descemet membrane may influence disease evolution.