We investigated the effects of diclofenac sodium and indomethacin on the proliferation of and collagen synthesis by lens epithelial cells (LECs) of human cataracts in culture. The anterior capsule with attached LECs, obtained by anterior capsulotomy during cataract surgery, was cultured directly without cell dispersion. When the culture became almost confluent, diclofenac sodium or indomethacin at various concentrations was added to the incubation medium. The incorporation of 3H-thymidine and 3H-proline into the LECs was measured after the cells were labeled with these radioactive materials. Both drugs greatly suppressed the incorporation of 3H-thymidine and of 3H-proline, indicating that they inhibit cell division and collagen synthesis by LECs. Both drugs may help prevent posterior capsule opacification.