Purpose: This study was undertaken to determine the performance of trabeculectomy surgery over a 20-year period and examine the associations between outcome and risk factors for trabeculectomy failure. Design: Retrospective cohort study. Participants: A total of 234 patients (330 procedures) who had undergone trabeculectomy surgery at Addenbrooke's Hospital, Cambridge, United Kingdom, between January 1988 and December 1990. Methods: Patients were identified through surgical logbooks (n = 521 procedures on 380 patients); after this, a case-note review was undertaken, which identified 234 patients (330 procedures) who had available case notes. Main Outcome Measures: Surgical success was defined as "complete success" while intraocular pressure (IOP) remained <21 mm Hg with no additional medication and as "qualified success" if those requiring additional topical medication were included. Functional success was defined if patients did not progress to legal blindness (visual acuity <3/60 or visual field <10 degrees). Results: After 20 years, 57% were classified as complete success, 88% were classified as qualified success, and 15% had become blind. Those at risk of trabeculectomy failure were younger or had uveitic glaucoma. Those with pseudoexfoliation or aphakia were more likely to progress to blindness. Furthermore, those using 2 or more topical medications or with advanced visual field loss at the time of surgery were more at risk of both trabeculectomy failure and blindness. Conclusions: This study indicates that trabeculectomy survival at 20 years may be approximately 60% with no topical medication and approximately 90% with additional topical medication. Patient age, preoperative topical medication use, glaucoma type, and glaucoma severity will independently influence this outcome. Trabeculectomy surgery is therefore a long-term solution to IOP control. Financial Disclosure(s): The author(s) have no proprietary or commercial interest in any materials discussed in this article.