AIMS/BACKGROUND--A previous study has shown that in age-related macular degeneration a high degree of concordance of disciform scar size occurs in the two eyes of any one patient. In a study of 35 patients with choroidal neovascular membrane who were treated with low dose ionising radiation to the macula of the affected eye, 11 were found to have bilateral disease. METHODS--The visual outcome and scar size and morphology in the two eyes of each of these patients were compared. RESULTS--In all radiotherapy treated eyes the appearance of the scar ranged from subtle subretinal pigmentary changes to dense subretinal fibrosis. By contrast all untreated fellow eyes showed marked subretinal scarring. Scars in radiotherapy treated eyes occupied an area that was approximately one third of that in untreated fellow eyes (3.8 mm2 v 11.7 mm2). Distance and near visual acuities in radiotherapy treated eyes were significantly better than that of untreated fellow eyes (p < 0.0033). Although untreated fellow eyes of necessity had longer follow up periods (64.5 months), the mean follow up time in treated eyes was 28 months by which time the disciform response is generally thought to have ceased evolving. CONCLUSION--This study has provided evidence in support of reduced scarring and maintenance of better central visual function in radiotherapy treated eyes when compared with untreated fellow eyes.