The purpose of this study was to compare the IOP fluctuations in the daytime tension curves of glaucoma patients under ocular hypotensive therapy with those of patients previously submitted to trabeculectomy. We also evaluated the IOP peaks and fluctuations for the same patients in response to the water-drinking test (WDT). The study included 30 primary open-angle glaucoma (POAG) patients using ocular hypotensive medications and with no history of previous intraocular surgery (medical group), and 30 POAG patients previously submitted to one or more trabeculectomies though taking no medication at the time of the study (surgical group). All patients were submitted to a diurnal tension curve--DTC (8:30-17:00/3-hour intervals) followed by the WDT. The IOP peak and IOP fluctuation during the diurnal tension curve were significantly greater in the medical group than in the surgical group. The same was observed following the WDT. From an overall baseline IOP of 10.6 mmHg, the mean IOP change following the WDT was 13% in the surgical group and 40% in the medical group. Patients submitted to trabeculectomy have less TOP fluctuations during the diurnal tension curve and following a water-drinking provocative test. This effect could represent an additional benefit of surgery in controlling the intraocular pressure of glaucomatous patients.