Background: This is the first comprehensive comparison between gonioscopy-assisted transluminal trabeculotomy (GATT) and GATT combined with ab interno canaloplasty (ABiC) in patients with OAG. Purpose: The purpose of this study was to compare the efficacy and safety of gonioscopy-assisted transluminal trabeculotomy (GATT) combined with ab interno canaloplasty (ABiC) with those of GATT alone in patients with open-angle glaucoma (OAG). Design: This was a retrospective, comparative case series. Participants: Patients with primary open angle glaucoma who underwent GATT (27 eyes in Group 1) or GATT + ABiC (26 eyes in Group 2) were included. Methods: Outcomes including intraocular pressure (IOP), glaucoma medications, and surgical complications were analyzed. Main outcome measures: Surgical success was defined in terms of IOP and medication use as follows: (1) a preoperative IOP > 21 mmHg and a postoperative IOP <= 21 mmHg with at least a 20% reduction from baseline with (qualified success) or without (complete success) glaucoma medications or (2) a preoperative IOP < 21 mmHg while taking 3 or more glaucoma medications and a postoperative IOP <= 21 mmHg with a reduction of more than two medications (qualified success) or with no medications (complete success). Results: At 12 months, the mean IOP was 14.8 +/- 2.2 mmHg in Group 1 and 16.6 +/- 2.3 mmHg in Group 2 (p = 0.008). The number of medications was 0.6 +/- 1.0 in Group 1 and 0.9 +/- 1.3 in Group 2 (p = 0.334). At 24 months, the mean IOP was 15.3 +/- 2.0 mmHg in Group 1 and 15.5 +/- 2.4 mmHg in Group 2 (p = 0.676). The number of medications was 0.5 +/- 0.9 in Group 1 and 0.9 +/- 1.1 in Group 2 (p = 0.197). The complete success rates were 63.0% in Group 1 and 50.0% in Group 2 (p = 0.16), and the qualified success rates were 81.5% in Group 1 and 76.9% in Group 2 (p = 0.51). Conclusion: The GATT procedure, with or without ABiC, is safe and effective in decreasing the IOP and the number of antiglaucoma medications used.