One hundred and sixteen patients with newly diagnosed primary open angle glaucoma were selected for a randomised, prospective, multicentre trial if the untreated intraocular pressure was over 25 mmHg and there was field loss characteristic of glaucoma. Conventional management of medical therapy followed by trabeculectomy in unsuccessful cases was compared with trabeculectomy at diagnosis followed where necessary by supplementary medical therapy. At a mean follow-up of 4.6 years there was no significant difference in visual acuity between the groups but the conventional management group had significantly greater loss of visual field which occurred during the unsuccessful attempt at medical control. The eyes which lost most visual field were those with least field loss at diagnosis and this paradox was attributed to a prolonged attempt at medical control in these eyes because they were thought to have a lower risk of visual field deterioration.