Purpose: To determine the incidence of glaucomatous progression at mean intraocular pressure (IOP) levels in patients with ocular hypertension (OHT). Methods: A retrospective, multicentre, cohort analysis of 230 OHT patients with 5 years of follow-up evaluated for risk factors associated with progressive optic disc and visual field loss to determine the incidence of glaucomatous progression. Results: Forty percent of patients with IOPs ≥24 mmHg, 18% of patients with IOPs of 21-23 mmHg, 11% of patients with IOPs with 18-20 mmHg, and 3% of patients with IOPs of ≤17 mmHg progressed to glaucoma. The mean IOP was 19.8±2.4 mmHg in the stable group and 21.7±2.6 mmHg in the progressed group (P=0.0004). The highest average peak IOP was 23.4±4.0 mmHg in the stable group and 25.2±3.1 mmHg in the progressed group (P=0.006). Based on the pachymetry values for central corneal thickness, patients with thinner corneas more often progressed to glaucoma (P<0.0001). A multivariant regression analysis to determine risk factors for progression was positive primarily for higher peak IOPs, older age, male gender, argon laser trabeculoplasty, visual acuity ≥20/50, and no topical medical therapy or β-blocker therapy prior to the study. Conclusions: IOP reduction within the normal range over 5 years of follow-up reduces the chance of progression to primary open-angle glaucoma in OHT patients. © 2009 Macmillan Publishers Limited All rights reserved.