The youngest affected member of a family with a five-generation history of posterior amorphous corneal dystrophy underwent penetrating keratoplasty. The corneal button was studied by light and electron microscopy, representing the first pathologic description of this condition. Light microscopy demonstrated fracturing of the most posterior collagen layers of the stroma and focal attenuation of endothelial cells. Electron microscopy showed the collagen fibers in the most posterior stromal lamellae to be disorganized. Descemet's layer was interrupted by a band of collagen fibers resembling stroma, and there was loss of endothelial cells. These findings suggest a developmental abnormality in the formation of the posterior stroma and Descemet's membrane in posterior amorphous corneal dystrophy.