The role of early surgery in the management of primary open angle glaucoma is under debate. To determine whether previous medical therapy influences the outcome of subsequent trabeculectomy, we retrospectively reviewed the results of surgery in two groups of patients. The first group underwent primary trabeculectomy, having had an average of 2 weeks of preoperative medical therapy, and this group was compared with a group of patients who had received at least 1 year of topical glaucoma therapy before undergoing trabeculectomy (the multiple-treatment group). The two groups were similar in terms of a number of variables, including race, age, sex, presenting intraocular pressures, and presenting visual fields, and they differed only in the known duration of their disease. The success rate of trabeculectomy was significantly higher in the primary trabeculectomy group as compared with that in the multiple-treatment group (P less than .001). We discuss the possible reasons for this difference and its implications for the future management of primary open angle glaucoma.