SUMMARY The result of a 10-year longitudinal study of the fellow eye at risk of developing closedangle glaucoma (because the presenting eye had the disease) is If the fellow eye that will develop closed-angle glaucoma could be identified before the event, unnecessary iridectomies would be eliminated. This paper describes the result of a 10-year longitudinal study the purpose of which was to devise a scheme for selecting the eye that needs an iridectomy and to observe the behaviour of the untreated fellow eye.