The lens epithelium of the human eye obviously plays a crucial role in the pathogenesis of primary and secondary cataracts. Four hundred nine flat preparations of human lens epithelium attached to the anterior capsule, obtained during surgical extracapsular cataract extraction (ECCE), were studied. Of these, 197 proved to be suitable for quantitative cell density determination and 93 for counting of the Barr bodies (sex chromatin). Barr bodies are, on the average, found in 54% of the lens epithelial cell nuclei of female patients and in 8% of male patients (P less than 0.001). The median for the cell density of all preparations was 4382 epithelial cells/mm2. Lens epithelium of patients exceeding 50 years of age showed a lower cell density than epithelium of younger patients (P less than 0.05), and, surprisingly, epithelium of females showed a significantly higher density than that of male patients (P less than 0.01). With increasing cortex involvement of the cataracts, a tendency towards a decrease of epithelial cells was noted.