The distribution and type of fiber cell damage was evaluated in human age-related nuclear cataracts and in aged normal (non-cataractous) lenses. Ten age-related nuclear cataracts (53 to 89 years old) and four normal lenses (59 to 67 years old) were examined by electron microscopy of fixed Vibratome sections. Images from the adult, juvenile, fetal and embryonic nuclear regions were compared. Each cataractous lens contained a central region of increased light scattering which involved the embryonic and fetal regions with progressively less involvement in the juvenile and adult nuclear regions. Some damaged fiber cells were observed in all specimens, although damage was minor and infrequent in the normal lenses. Degeneration of single or groups of fiber cells was noted in all the adult nuclei of the cataractous lenses, becoming less frequent in the juvenile nuclei. The types of damage included localized voids, multilamellar membrane aggregates, globular bodies, enlarged cells and regions of highly convoluted membranes. The fetal and embryonic nuclei of the cataractous lenses exhibited rare and minor morphological defects, and were virtually identical to the equivalent regions of the normal aged lenses. Examination of cell interfaces in opaque regions of cataractous lenses revealed that the oldest fiber cells sustained apparent membrane loss. Extracellular spaces in the embryonic, fetal and juvenile regions of the cataractous lenses often contained dense deposits, presumably cytoplasmic material lost from adjacent fibers. The results indicate that the region of greatest nuclear opacity, located in the lens center, does not contain any significant cellular damage. This suggests that older fiber cells respond differently to pathological and senescent changes than younger cells made after fetal development. The observed loss of membranes and cytoplasmic material from the oldest fiber cells may be a contributory mechanism in the formation of age-related human nuclear cataracts.