Trabeculectomy reduces intraocular pressure by fistula formation into a subconjunctival bleb. The operation site traditionally has been at the superior corneo-scleral junction with a failure rate of 10-30%. The causes of trabeculectomy failure can be broadly classified into intraocular, scleral and extraocular. Extraocular factors account for the majority of failures, the main cause being increased subconjunctival fibrosis. Hitherto the effect of varying the site of filtration surgery on intraocular pressure control has not been studied. We therefore carried out a prospective study to evaluate the optimum site for trabeculectomy. Sixty patients were randomised to undergo a superior, nasal or temporal trabeculectomy. Patients who underwent a nasal trabeculectomy had significantly lower intraocular pressures 18 months after surgery (p < 0.05), with 30% having an intraocular pressure of < or = 10 mmHg.