A modification of the trabeculectomy technique was designed in an attempt to increase the outflow of aqueous into the subconjunctival space in the early post-operative period. This 'high flow' method was compared with a control group of eyes undergoing a standard trabeculectomy. Fifty-three eyes of 44 patients were recruited to this study. Follow-up was for a minimum of 12 months (mean 20 months). The high flow method resulted in lower intraocular pressures (IOP) on days 1 and 2 and shallower anterior chambers without increased incidence of conjunctival wound leak, suggesting that high outflow was indeed achieved. However, long-term control of IOP was not better than that achieved by the conventional technique. IOP control (IOP < 21 mmHg) was achieved in 64.2% of eyes (34 eyes) without additional topical medication. IOP control with topical medication was satisfactory in the remaining 19 eyes, and no patient required further drainage surgery. Further analysis of the data showed that the most important factor determining success or failure of trabeculectomy in controlling IOP was the duration and number of prior topical medications. The success rate for trabeculectomy was significantly higher in those eyes with a shorter duration and quantity of prior topical therapy.