Purpose: To evaluate whether patients who have undergone trabecu- Iectomy with mitomycin C have a different intraocular pressure (IOP) re- sponse profile after the water drinking test (WDT) from that of patients who are medicakky managed with a similar baseline IOP and level of uisu-all field (VF) damage. Design: Prospective observational study. Participants: 12 glaucoma patients with IOP controlled by trabeculec- tomy and 12 with medically treted glaucoma matched for level of VF damage and IOP at baseline (7-14 mmHg). Methods: All patients underwent the WED, which involved drinking 1000ml of water in 15 minutes. Main outcome measures: The IOP was measured before the WDT and subsequently at 15-minute intervals for I hour. The peak IOP with the WDT was compared between both groups using paired tests. Results: Baseline IOPS were 10.3±2.2mmHg in surgically trated and 11.1± 1.7 mmHg in medically treated eyes. After the WDT, mean IOPS in the surgically and medically treated groups were 10.6±2.1 mmHg and 15.1±2.4mmHg. Mean ,aximum IOPS were 11.6±2.4 mmHg and 17.2±2.3mmHg in the surgically and medically treated groups, respectively (p < 0.0001),increases of 17.6% and 54.9%. Ranges of IOP during the WDT were 2.2 ± 1.2 mmHg and 7.5±1.8mmHg in the surgically and medically treated patients (p < 0.0001).