PURPOSE: To provide 4-year data on the efficacy and safety of gonioscopy-assisted transluminal trabeculotomy (GATT) in patients with open-angle glaucoma. DESIGN: Retrospective case series. PARTICIPANTS: Eyes of patients > 18 years of age who underwent GATT by a single surgeon at Wills Eye Hospital with at least 36 months follow-up. METHODS: Postoperative changes in outcome measures including intraocular pressure (IOP), medication use and visual acuity were recorded. Failure was defined as IOP > 21 mmHg or less than 20% reduction below baseline at any postoperative visit after 3 months or need for further glaucoma surgery. MAIN OUTCOME MEASURES: Main outcome measures were failure rate, IOP, number of glaucoma medications, and visual acuity at 4 years. RESULTS: Fifty-nine patients (74 eyes), age 57.1 ± 18.5 years (37.8% female) underwent the GATT procedure. Average follow-up was 47.0 ± 6.7 months (range 35.6-76.5 months). Mean IOP was 27.0 ± 10.0 mmHg preoperatively and 14.8 ± 6.5 mmHg at 4 years (45% IOP decrease; P < 0.01). Mean number of medications decreased from 3.2 ± 1.0 preoperatively to 2.3 ± 1.0 at 4 years (P < 0.01). The cumulative failure rate at 4 years was 53.9%, and the cumulative reoperation rate was 42.0%. No significant differences between patients with primary open-angle glaucoma and other types of glaucoma were found. CONCLUSIONS: Gonioscopy-assisted transluminal trabeculotomy can be a safe and effective conjunctival-sparing surgery for treating various forms of open-angle glaucoma at 4 years. FINANCIAL DISCLOSURE(S): Proprietary or commercial disclosure may be found after the references.