PURPOSE: To compare the intraocular pressure (IOP) diurnal fluctuations of glaucoma patients treated with latanoprost 0.005% once a day with patients with controlled IOP after deep sclerectomy or trabeculectomy. METHODS: The trial included 60 prospectively recruited subjects with primary open-angle glaucoma. The medical group consisted of 20 patients with controlled IOP (<18 mm Hg) under latanoprost 0.005% monotherapy and with no history of previous intraocular surgery or argon laser trabeculoplasty; the surgical groups included 20 patients after trabeculectomy, and 20 patients after deep sclerectomy with collagen implant (DSCI). The patients in the surgical groups had a controlled IOP without any ocular hypotensive medications. All patients underwent a diurnal tension curve (08:00-17:00/three-hour intervals), followed by a water-drinking test (WDT) with the last IOP measurement taken at 21:00 hours. The between-group differences were tested for significance by means of analysis of variance (ANOVA). RESULTS: Baseline IOP was significantly different between the trabeculectomy group (10.1 mm Hg (3.4 SD)), the DSCI group (13.9 mm Hg (2.8)) and the latanoprost group (15.5 mm Hg (2.0); p = 0.005). The average IOP during the diurnal tension curve (10.1, 13.7, and 15.7 mm Hg, respectively, for the trabeculectomy, DSCI, and latanoprost groups) differed significantly between groups (ANOVA; p<0.0001), but the variation was comparable in the three groups (ANOVA; p = 0.13). After the WDT, elevation of IOP was significantly greater among patients treated with latanoprost (p = 0.003). CONCLUSION: Trabeculectomy patients had a statistically significant lower average IOP in the diurnal tension curve compared with the other two groups. No wider variation in diurnal IOP with latanoprost compared with the surgical procedures was found. The IOP increase during the WDT was most marked in patients under latanoprost therapy.