OBJECTIVES: To investigate the correlation and agreement between the intraocular pressure peaks detected during the water drinking test and the modified diurnal tension curve in untreated glaucomatous eyes. INTRODUCTION: It has been suggested that the intraocular pressure peaks detected during the water drinking test predict the peaks observed during a 24-hour diurnal tension curve. A more feasible and practical test for assessing intraocular pressure peaks in glaucomatous eyes during office hours would be of great clinical utility. METHODS: This was a prospective study involving open angle glaucoma patients without anti-glaucoma medication submitted to the modified diurnal tension curve and water drinking test on the same day. The intraocular pressure peaks during the water drinking test and the modified diurnal tension curve were reported and compared. Statistical analysis was performed to assess the correlation and agreement between intraocular pressure peak measurements. RESULTS: The correlation between intraocular pressure peaks during the water drinking test and modified diurnal tension curve was significant and strong (Pearson's Correlation Coefficient r=0.780, p<0.0001). Limited agreement was observed between these measurements. Eighty-two percent of intraocular pressure peaks were higher during the water drinking test than the modified diurnal tension curve. DISCUSSION: These findings suggest that the water drinking test may be used as tool to assess risk factors for glaucomatous patients. CONCLUSION: Intraocular pressure peaks detected during the water drinking test could be used in clinical practice to both estimate the peaks observed during the modified diurnal tension curve and assess the status of the eye's outflow facility.