In a randomized, controlled study drops of 1% aqueous solution of indomethacin were compared to Placebo to assess efficacy in the prevention of cystoid macular edema. The study involved 124 patients who had undergone intracapsular cataract extraction and 40 patients with implanted lenses. The study parameters--visual acuity, biomicroscopy observations of the macula and fluorescein angiography--were assessed for three months postoperatively. The long-term course was reviewed by regular outpatient follow-up visits or by questionnaires completed by the patients' personal ophthalmologists. Fluorescein angiography could only be carried out in 13 of the 40 patients who had intraocular lenses. The relatively high incidence of secondary cataract or pupillary synechiae made technically perfect exposures difficult. In 73 patients with intracapsular extraction, fluorescein angiography showed a distinct diminution in the incidence of edema in the indomethacin group at all three observation periods (3, 6 and 12 weeks postoperatively). The difference compared with placebo was significant at weeks 6 and 12. Ophthalmoscopically visible macular changes were rare. No evidence of edema was noted in any of the 51 patients who could not have fluorescein angiography for one reason or another. The visual acuity of patients who received indomethacin was not significantly different from that of those who received placebo. Furthermore, and this is important for the patients, in the group with proven edema no conclusion could be made about the effect of therapy on the severity and persistence of visual impairment.(ABSTRACT TRUNCATED AT 250 WORDS)