BACKGROUND Posterior polymorphous dystrophy of the cornea is an uncommon, but well-recognized, congenital disease affecting Descemet's membrane and endothelium. Visual impairment usually is minimal and slowly progressive. Edema develops in a small number of patients, necessitating a corneal graft in middle age. An early manifestation of posterior polymorphous dystrophy with corneal edema present at birth or soon thereafter is extremely rare; only a few ultrastructural reports are available. METHODS The corneal button of a 5-month-old baby with posterior polymorphous dystrophy was examined by conventional light and electron microscopy. RESULTS The anterior banded zone of Descemet's membrane was thinned (2 microns) in the axial part of the cornea, contained defects in the paraxial part, and was absent at the periphery. Where present, the anterior banded zone was lined posteriorly by a fibrocellular layer (containing fibroblast-like spindle cells and striated collagen fibrils) and a fibrillar layer forming an abnormal posterior collagenous layer. This layer was lined posteriorly by degenerate overlapping endothelial cells, which also were in apposition to the stroma in regions with an absent anterior banded zone. The defects in Descemet's membrane were used by the abnormal keratocytes to migrate between the anterior banded zone and the posterior collagenous layer. At the periphery, a multilayer of spindle cells was present behind the stroma. CONCLUSIONS Failure to produce a continuous anterior banded zone indicates an onset of the disease before the twelfth week of gestation. It appears that, in posterior polymorphous dystrophy, there is a mosaic of better preserved and dystrophic endothelial cells and that the presence or absence of the normal components of Descemet's membrane is determined by the proportion of dystrophic cells in the endothelial cell population.