Transmission electron microscopy studies of cornea, iris and lens from a case of ocular siderosis due to a retained intraocular foreign body (IOFB) and vitreous hemorrhage are presented. We observed widespread degeneration of lens epithelium, iris stromal cells and iris pigment epithelium associated with intra- and extracellular siderosomes. A direct relationship was found between the degree of cell degeneration and the number of intracellular siderosomes in lens epithelium. This is contrary to findings in other siderotic tissues which show intracellular iron deposition to be associated with increased ability to detoxify iron. The cornea showed siderosomes only within keratocytes, and there was no evidence of a relationship between siderosome accumulation and cell degeneration. The literature on ultrastructural studies of ocular siderosis is reviewed and the current theory of the mechanism of chronic iron toxicity is discussed.