Four cases of epithelial downgrowth following cataract surgery were studied by electron microscopy. The invading epithelium was excised by posterior keratectomy, iridectomy, and vitrectomy. Diagnostic photocoagulation on the iris was performed before the operations in all cases. Multilayered epithelium on these tissues showed ultrastructural features similar to conjunctival epithelium, and in the cornea no intact endothelial cells were identified. A subepithelial connective tissue of varying grades of development was present between the downgrowth epithelium and Descemet's membrane. The basal epithelial cells on the vitreous contained numerous dense granules, and a thick basement membrane-like layer was found underneath. Changes attributable to photocoagulation included partial replacement of the iris pigment epithelium with macrophages, degeneration of blood vessels, and marked swelling and dissociation of collagen fibrils in the iris stroma due to splitting into longitudinal filamentous components. Identical changes of collagen fibrils could be reproduced in the rabbits iris after experimental photocoagulation.