The ultrastructure of human diabetic lens nuclei is described for the first time. Two cataractous lenses from late-onset diabetics were examined using transmission electron microscopy to determine the type and distribution of cellular disruptions. The diabetic lens nuclei were compared to a transparent nucleus from a normal human lens. Cellular damage to the exterior region of the diabetic lens nuclei was extensive, especially at the cortical-nuclear interface. Areas of lens fiber condensation as well as areas of cytoplasmic loss were observed in the outer nucleus. Morphological defects commonly seen in this region included: multilamellar membrane aggregates, voids where cytoplasmic material was lost, deposits in the extracellular spaces, density variations between adjacent fiber cells, and heterogeneously staining globules. The opaque central regions of the nuclei displayed relatively little cell damage, but fiber cells were very irregular in shape and packing. The ultrastructure of inner nuclear fiber cells was comparable to that seen in the normal lens and in age-related nuclear cataracts in non-diabetics. It appears that the effect of hyperglycemia on lens fiber cells is dependent on their age and stage of differentiation.