We studied the transmission electron microscopy of the abnormal collagenous tissue posterior to Descemet's membrane in 30 human corneas that represented 19 different diseases. We named this abnormal tissue the "posterior collagenous layer" of the cornea and classified it into three morphologic types: (1) banded posterior collagenous layer, characterized by 110-nm wide-spacing material, (2) fibrillar posterior collagenous layer, characterized by loose meshwork of collagenous fibrils 20 to 30 nm in diameter, and (3) fibrocellular posterior collagenous layer, characterized by fibroblasts in connective tissue. The published ultrastructural studies of 88 corneas that represented 13 different clinical disorders fit this classification well. The posterior collagenous layers often formed multiple lamellae, but no pattern was invariably associated with a specific clinical disorder. We think these multiple layers represent successive periods of deposition of collagen by a stressed corneal endothelium and provide a lifelong record of senescent and pathologic events.