• Thirteen human lenses with posterior subcapsular cataracts were examined by transmission electron microscopy to study the lens epithelial cells that characteristically migrate posteriorly in this disease. A sequence of histologic changes was identified from the equatorial region to the posterior pole. The cells became increasingly more active cytologically, culminating in lens-fiber-like formation or cell death at the posterior pole. Moderate amounts of extracellular granular and fibrillar material were produced, especially in the more advanced cases. No mature collagen was found. The liquefied posterior cataracts contained cellular debris from both degenerate lens fibers and necrotic migrating cells. The migrating lens cells thus contributed to the cataract by secretion, cytolysis, and probably by release of lysosomal enzymes. Bladder cells had many of the characteristics of lens fibers so that the term "abortive" or dysplastic lens fibers is appropriate for them.