• The ultrastructure of Descemet's membrane was studied by transmission electron microscopy in corneal buttons removed from 11 phakic eyes with Fuchs' dystrophy. Abnormalities in Descemet's membrane consistent with abnormal endothelial function early in life (prior to age 20 years) were present in all corneas. Thus, despite the relatively late clinical onset of Fuchs' dystrophy, endothelial abnormalities are present quite early in life in this disease. An abnormal fibrillar layer was thicker in those corneas with greater stromal and epithelial edema, possibly indicating that this layer is formed mainly during periods of endothelial decompensation.