We investigate the efficacy of triple-fixed-combination of bimatoprost/brimonidine/timolol once and twice a day, demonstrating higher intraocular pressure reduction with once-a-day use, and discuss possible implications based on previous literature. The purpose of this study was to compare the efficacy of a fixed combination bimatoprost-timolol-brimonidine (Triplenex) instilled once-daily with twice-daily in primary open angle glaucoma. A randomized clinical trial at a public eye care institution. Thirty patients with primary open angle glaucoma were followed up for 3 months. The right and left eyes of these patients were randomly assigned to once-daily (10:00  pm ) or twice-daily (10:00  am ; 10:00  pm ) regimens of fixed combination bimatoprost-timolol-brimonidine. Intraocular pressure peaks were obtained with the water drinking test before the introduction of this medication (basal WDT0), 1 month (WDT1), 2 months (WDT2), and 3 months (WDT3) after starting the use of the fixed combination of bimatoprost/timolol/brimonidine (Triplenex). Variation from peak intraocular pressure at WDT3 to peak at WDT0 was compared within groups of 2 versus once-daily regimen. Sixty eyes of 30 patients (age: 70.67±9.70 y) were included in this study. Baseline clinical characteristics were comparable between groups. The mean reduction in peak intraocular pressure from WDT0 to WDT3 was 6.1±6.1 mm Hg (30.5%) in the eyes receiving 1 drop per day and 4.3±5.7 mm Hg (21.5%) in the eyes receiving 2 drops per day ( P =0.023). Mean reduction in peak intraocular pressure considering all WDT was also higher in the group with once-a-day treatment (5.90±6.03 vs. 4.46±4.28 mm Hg, P =0.006). Fixed combination of bimatoprost 0.01%, brimonidine tartrate 0.15%, and timolol maleate 0.5% once-a-day is more effective in reducing peak intraocular pressure as measured by the WDT than twice-a-day dosing.