A double-blind study was performed on 64 patients to assess the effect of preoperative indomethacin in comparison with a placebo in preventing surgically induced miosis during extracapsular cataract extraction. One drop of indomethacin or placebo was instilled at 7.00 p.m. and at 9.00 p.m. the night before surgery and 2 hours, 1 hour, and 15 minutes preoperatively. The pupil diameter was recorded at different stages of the operation. It was found that indomethacin as compared with the placebo significantly reduced the amount of pupil constriction during the operation, making the removal of lens material and the implantation of an intraocular lens easier. The indomethacin is thought to act by inhibiting prostaglandin synthesis caused by iris trauma during surgery.