Purpose. The goal of this study was to compare the ultrastruc-ture of the oldest cells in opaque and transparent human lenses.Methods. Age-related nuclear cataracts, late-onset diabetic nuclear cataracts and normal aged lenses were examined by transmission electron microscopy. Cross-sectional profiles of fiber cells in the embryonic, fetal and juvenile nuclear regions were obtained to facilitate direct comparisons between lens regions and between sample groups. Image analysis was performed to determine cross-sectional areas of fiber cells in each region.Results. The average cross-sectional area increased approximately sixfold from the outer to the inner nuclear regions in all lenses measured. In each nuclear region, fiber cells displayed a characteristic size, shape, arrangement and type of interdig-itations which were consistently seen in all the lenses examined. Some lenses had more complex interdigitations than others. Gap junctions were identified as pentalamellar structures having 16 nm width and appeared identical throughout the nuclei of both normal and cataractous lenses. The cytoplasm of all lenses was smooth and free of large density variations. However, the cytoplasm of some cataractous lenses appeared more granular in texture than noncataractous lenses. Cellular degeneration, debris or large cellular defects were not seen in the cores of cataractous lens nuclei.Conclusions. These results indicate that only minor ultrastruc-tural differences exist between the oldest fiber cells in normal and cataractous lenses, and that the presence of extensive cellular damage and disruptions is not necessary for the generation of nuclear opacities in aged lenses. Our observations suggest that light scattering sufficient for vision impairment may involve structural alterations much smaller than previously proposed.