Precis: The present study demonstrates that surgical success rate of gonioscopy-assisted transluminal trabeculotomy (GATT) is 83.7% according to target intraocular pressure (IOP) <= 21 mm Hg and >= 20% reduction from baseline in patients with moderate to advanced glaucoma, with an average follow-up of 19.4 months. Purpose: The aim of the study was to assess the outcomes of GATT in eyes with moderate to advanced open-angle glaucoma. Patients and Methods: A retrospective study included 104 eyes of 104 patients, with a mean age of 58.9 +/- 18.5 (15 to 87) years, who underwent GATT using the 6/0 prolene suture to treat open-angle glaucoma. The GATT procedure was carried out alone or in combination with phacoemulsification. IOP at baseline, third, sixth, ninth, 12th, and 18th months of follow-up; need of antiglaucoma medication; need of further glaucoma surgery; cup/disc ratio; and best-corrected visual acuity (BCVA) were recorded. Patients with prior incisional glaucoma surgery were excluded. Surgical success was determined for both IOP Results: A total of 65 patients with primary open-angle glaucoma (POAG) and 39 patients with secondary open-angle glaucoma (SOAG), including pseudoexfoliative, pigmentary, uveitic, and steroid-induced glaucoma underwent GATT. Preoperative mean IOP was 25.0 +/- 7.3 mm Hg. Preoperative average number of antiglaucoma medications was 3.4 +/- 0.6. The average follow-up time was 19.4 +/- 8.1 (6 to 37) months. Preoperative average BCVA was 0.51 +/- 0.24, and cup/disc ratio was 0.85 +/- 0.14. Surgical success was achieved in 87 of 104 (83.7%). Seven patients had medically uncontrolled glaucoma after GATT surgery, and they underwent further surgery. Patients with POAG had a higher percentage of IOP reduction than patients with SOAG at 18th month of follow-up (40.1% vs. 27.6%). Need for medication at final visit was similar between POAG and SOAG. Conclusion: GATT procedure is a safe and successful option for the treatment of moderate to advanced open-angle glaucoma. Surgical success could be maintained up to similar to 18 months.