Purpose: To prospectively study prognostic factors for normal-tension glaucoma (NTG) under treatment with topical beta-blocker. Methods: One hundred and forty-six eyes of 146 patients with NTG with a mean untreated intraocular pressure (IOP) of 14 mmHg, mild to moderate visual field damage and mean spherical equivalent refraction of -3.5 (-8.0 to +2.0) dioptre were randomized to topical nipradilol or timolol and followed for 3 years. The Humphrey full threshold 30-2 visual field test was performed every 6 months, and optic disc photographs were obtained every 12 months. Progression was defined as visual field progression, optic disc and/or peripapillary nerve fibre layer change, and factors relating to progression were evaluated using Cox proportional hazards models. Results: IOP decreased by 1.0 mmHg over the 3-year period, during which 35% showed progression according to the aforementioned criteria. Optic disc haemorrhage (hazard ratio [HR] 4.00, p < 0.001) and less extent of myopia (per dioptre, HR 1.15, p = 0.013) were significant risk factors. When progression was defined by visual field progression only, less extent of myopia was again a significant risk factor (HR 1.17, p = 0.038). Conclusion: Beside optic disc haemorrhage, less extent of myopia was a risk factor for progression in the current NTG population where most patients were mildly myopic and IOP during follow-up averaged 13.2 mmHg under topical beta-blocker.