OBJECTIVE To evaluate whether the amplitude of day-and-night intraocular pressure (IOP) profiles influences the rate of progression of chronic open-angle glaucoma. METHODS The hospital-based clinical observational study included day-and-night profiles of IOP measurements performed on 458 patients (855 eyes) with chronic open-angle glaucoma or ocular hypertension. The 24-h pressure profiles obtained by Goldmann applanation tonometry contained measurements at 0700, noon, 1700, 2100, and midnight. RESULTS In the whole study population, IOP amplitude was significantly (P<0.001) and positively associated with the mean (r=0.26), minimal (r=-0.23) and maximal (r=0.59) IOP values. Taking the whole study population, glaucoma progression was not associated with the IOP amplitude (P=0.09). After adjustment for age, neuroretinal rim area and the other IOP measurements, age (P<0.001) and neuroretinal rim area (P=0.05) remained as significant predictive factors in the selected Cox model. In the normal-pressure glaucoma group (n=174 eyes), progression was significantly positive associated with the minimal IOP value (P<0.001), the mean of the IOP values (P=0.024), and, less significantly (P=0.037) and negatively, with the pressure profile amplitudes. In the high-pressure glaucoma group (n=681 eyes), rate of glaucoma progression was not associated with the IOP amplitude (P=0.734) or the other IOP parameters. CONCLUSIONS Taking into account the highly significant associations between the IOP amplitude and the mean, minimal and maximal IOP values suggests that it is the IOP itself, and less the pressure amplitude, which has the main influence on the rate of the glaucoma progression.