The morphology, cell density and mitotic index of human lens epithelial cells were determined according to the region of the opacity as follows: (1) nuclear; (2) posterior subcapsular; (3) cortical; (4) nuclear and posterior subcapsular; (5) cortical and posterior subcapsular. Epithelial abnormalities, consisting of enlarged, polymorphous cells and intercellular clefts, were found in cataractous lenses involving posterior subcapsular and/or cortical opacities. In contrast to the normal pattern of cell density, the cell density of cataractous epithelia in these categories decreased from the central to the peripheral epithelium. This decrease in cell density was due to the presence of swollen cells and intercellular clefts. The central and pre-equatorial mitotic index of cataractous lens epithelia was not significantly different (range 0.002-0.008%) and no correlation was found between cataract type and mitotic index (no information was available for the equatorial region). The low mitotic index observed reflects the mature age range of cataractous lenses and in one normal lens epithelium from a 23-yr-old donor, a higher value of 0.015% was found. The relevance of these epithelial abnormalities to the normal function of the epithelium is discussed.