Corneal blood staining represents deposition of hemoglobin and its breakdown products within the cornea. Pathologic examination of these corneas typically reveals degenerating endothelial cells and keratocytes. These degenerative changes have typically been attributed to the blunt trauma itself or to toxicity of the erythrocytic debris (a "localized hemosiderosis"). Another possible mechanism for this injury, however, is porphyrin-induced photosensitivity. Examination of frozen sections of an acutely blood-stained human cornea demonstrated fluorescence within all layers of the cornea, similar to that seen with hematoporphyrin derivative. The production of cytotoxic oxygen species within the blood-stained cornea exposed to light may contribute to endothelial and keratocyte degeneration. Limiting light exposure of blood-stained corneas or eyes with hyphemas might theoretically reduce light-induced and porphyrin-mediated toxicity.