Purpose : This study was designed to uncover a new sensitive and specific factor for predicting the progression of glaucoma. Methods : The 24-hour ambulatory blood pressure and diurnal curve of intra-ocular pressure were recorded in seventy patients: 51 primary open angle glaucoma (POAG) and 19 normal tension glaucoma (NTG). The mean systolic, diastolic and average arterial blood pressure were calculated, along with the nocturnal dip of systolic pressure and diastolic blood pressure. Two-year disease progression was assessed for all patients by means of retrospective analysis of visual fields defects on repeated perimetries. Results : Abnormal (absence or increased) nocturnal dip of systolic blood pressure was found to be correlated with disease progression in POAG and NTG patients with a sensitivity of 86% and a specificity of 85%, whereas no significant correlation was found for the other risks factors envisaged. Furthermore, a significant relationship between stable visual field defects and the use of diuretics/laser procedure was evidenced. Conclusion : The nocturnal dip of systolic blood pressure should be considered as a predictive factor of disease progression in NTG and POAG. Further prospective studies are needed to ascertain whether dip normalization could help slow down the visual field loss in these patients.