The incidence of pupillary fibrin membrane formation, a relatively frequent complication of cataract surgery in Japan even for experienced surgeons, was noted in 2038 consecutive cases of senile cataract removal and primary posterior chamber intraocular lens (PC IOL) implantation. The overall incidence was 4.4%. Comparisons among cases showed the incidence to be significantly higher among: (1) patients who received dry- versus wet-sterilized lenses (P less than 0.001), (2) eyes with premature, subcapsular, or nuclear cataracts than among those with mature cataracts (P less than 0.05), (3) patients with systemic hypertension (P less than 0.01), (4) eyes not treated postsurgically with topical antiprostaglandins (P less than 0.001), and (5) patients who had undergone previous PC IOL implantation in the fellow eye (P less than 0.001). These findings suggest that the formation of the fibrin membrane is a kind of immunologic reaction. Accordingly, use of antiprostaglandin agents and complete removal of lens material and epithelial cells are recommended, especially for patients undergoing second-eye operations or those with a predisposition to breakdown of the blood-aqueous barrier.