Purpose: The purpose of this study was to report 2 cases experiencing a transient decrease in their vision due to the development of acute myopia after an uneventful prolene gonioscopy-assisted transluminal trabeculotomy (GATT) surgery. Methods: A 52-year-old woman with primary open-angle glaucoma and an 8-year-old boy with juvenile glaucoma underwent uneventful GATT surgery. Results: On the postoperative first day, both patients had mild shallow anterior chamber and intraocular pressures measured as 19 and 16 mm Hg, respectively. Both patients had myopia measured as -4.5 and -6.0 D, respectively. Fundus examination was unremarkable in the first patient, whereas it showed bilateral optic disc cupping and retinal nerve fiber layer atrophy in the second patient. In the first patient, supraciliary effusion was detected by using ultrasound biomicroscopy. Ultrasound biomicroscopy could not be performed in the second patient, but ocular ultrasonography revealed a thin layer of suprachoroidal fluid. Myopia was completely regressed, and visual acuity increased within 1 week of the follow-up in both patients. At the sixth month of the follow-up, intraocular pressures in both patients were under control without any antiglaucoma medications. Conclusion: Transient decrease in vision after the GATT surgery might occur secondary to supraciliary effusion leading to acute, transient myopia.