That accelerated heavy ions are highly cataractogenic is indisputable. The basis of heavy particle effectiveness and the augmentation by fractionation, as recently demonstrated, remains less clear. There is no question, however, that these are tied to the ‘track structure’ and densely ionizing nature of the radiation. The unique energy deposition characteristics relating to charge and track structure are now being explored to begin to dissect the cellular response of the lens epithelium to radiation exposure. The elucidation of the basis of the cataractogenic effect of accelerated heavy ions is important not only to risk assessment, but also in the consideration of theories of radiation action and the mechanism of cortical opacification arising from a myriad of cataractotoxic agents.