The use of topical indomethacin in the prevention of surgically induced miosis has been documented. However, in these previous prospective trials this prostaglandin synthetase inhibitor was administered the day before surgery. With the frequency of 'day case' cataract surgery increasing, an efficient preoperative mydriatic regimen is important. In this study we considered the use of topical indomethacin as an addition to a regimen already implemented. One hundred and fourteen eyes underwent intercapsular cataract surgery, of which 64 were randomised to receive topical aqueous indomethacin one hour beforehand, and 50 eyes, which did not receive indomethacin formed the comparison group. Topical indomethacin reduced the miosis which occurs during cataract surgery whether performed under local or general anaesthesia. The operating time was shorter for eyes with less surgically induced miosis.