AIM: To study the possibility of using intraocular pressure (IOP) in the first postoperative day after sclerectomy as a prognostic indicator. METHODS: Non-randomised prospective trial involving 105 eyes of 105 patients with medically uncontrolled primary and secondary open angle glaucoma. Visual acuity, IOP, and slit lamp examinations were performed before and after surgery at 1 and 7 days, and 1, 3, 6, 9, 12, 18, 24, 30, 36, 48, 54, 60, and 66 months. Visual field examinations were repeated every 6 months. A split point on day 1 IOP of less than or equal to 5 mm Hg (61%) versus more than 5 mm Hg (39%) was used. The first postoperative day IOP was examined in relation to the need for subsequent Nd:YAG goniopuncture, the subsequent use of postoperative antiglaucoma medications, and as a stratification variable in the Kaplan-Meier analyses. RESULTS: The mean follow up was 43.2 (SD 14.3) months. The mean preoperative IOP was 26.8 (SD 7.7) mm Hg; the mean postoperative IOP was 5.1 (3.3) mm Hg at day 1 and 11.8 (3.1) mm Hg at month 60. Patients with IOP </=5 mm Hg had significantly fewer Nd:YAG goniopunctures (p = 0.0478). A significant (log rank test 0.0122) improvement for those with IOP </=5 mm Hg in terms of survival was detected using the most stringent criterion (IOP </=15 mm Hg with no medications). For patients with first postoperative day IOP </=5 mm Hg, the median time to failure was 24 months (95% CI: 12 to 30), but for those with an IOP >5 mm Hg, the median time to failure was only 6 months (CI 2 to 9). No significant difference in postoperative antiglaucoma medications was observed. CONCLUSION: First postoperative day IOP can be considered to be a significant prognostic indicator in deep sclerectomy.