The water drinking test (WDT) was once frequently used as a diagnostic tool for glaucoma, but not so often nowadays. In this study, we investigated the potential use of the WDT on the evaluation of trabeculectomy patency. Twenty age-matched volunteers and thirty-six glaucoma patients who were to receive trabeculectomy procedure were enrolled in this study. The WDT was given to the volunteers once and to all glaucoma patients before undergoing trabeculectomy and at certain intervals after the procedure. The WDT was performed in a standard manner. We defined four parameters after performing the WDT: Initial Pressure [IP, intraocular pressure (IOP) level before the WDT]; Slope of Ascending Trend (SOAT, the slope between baseline IOP and the highest IOP level); Peak Pressure (PP, the highest IOP level during the WDT); and End Pressure (EP, the IOP level after the WDT). It was found that the results of the WDT and trabeculectomy patency were strongly correlated. The four parameters in success and failure cases were significantly different at the last follow up: IP: (15.2 +/- 3.6 vs. 25.3 +/- 6.4, p<0.01); SOAT: (0.9 +/- 0.3 vs. 1.8 +/- 1.2, p<0.01); PP: (19.2 +/- 6.4 vs. 39.5 +/- 12.2, p<0.01); EP: (15.5 +/- 4.8 vs. 29.4 +/- 8.2, p<0.01). Thus, it was observed in this study that WDT was not only easy and safe to perform, but also valuable in evaluating the patency of trabeculectomy.