Introduction: To assess the effect of phacoemulsification and endo-cyclophotocoagulation (phaco-ECP) on intraocular pressure (IOP) fluctuation as assessed by the water drinking test (WDT) in primary open angle glaucoma (POAG). Methods: This was a prospective observational study carried out at a tertiary referral center. POAG patients on topical antiglaucoma medications and planned for phaco-ECP were recruited. WDT was performed before surgery and 6 weeks postoperatively by drinking 10 mL/kg of water in 5 minutes followed by serial IOP by Goldmann applanation tonometry measurements at 15, 30, 45 and 60 minutes. Mean IOP, IOP fluctuation (difference between highest and lowest IOP), IOP reduction, and factors affecting IOP fluctuation were analysed. Results: Twenty eyes from 17 patients were included. Baseline IOP was similar before (14.7 ± 2.7 mmHg) and after (14.8 ± 3.4 mmHg, p = 0.90) surgery. There was no difference in mean IOP (17.6 ± 3.4 mmHg vs 19.3 ± 4.7 mmHg pre and postoperative respectively, p = 0.26) or peak IOP (19.37 ± 3.74 mmHg vs 21.23 ± 5.29 mmHg, p = 0.25), albeit a significant reduction in IOP-lowering medications (2.2 ± 1.15 vs 0.35 ± 0.93, p

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