Context: POAG is a leading cause of irreversible blindness worldwide. Factors associated with the damage, progression of the disease and related blindness are poorly understood. Objective: To determine characteristics of patients with POAG that are associated with either a higher risk for blindness or preservation of visual function. Design: Prospective observational comparative cohort study. Participants: In all, 487 (974 eyes) consecutive incoming patients with POAG were followed for 5.5 3.6 years. Exclusion criteria were l) secondary glaucoma, 2) ocular conditions that would interfere with proper diagnosis and management, and 3) less than 3 months follow-up. Methods: Seventy-seven pieces of information were collected on each patient and updated at every six monthly visit. Comparisons were done between 1) patients with no legal blindness (NLB) and those with legal blindness (LB); 2) stable NLB patients and those who progressed; 3) the affected eye and non-affected eye of patients with unilateral blindness; and 4) initial presenting data of new patients. Setting: Academic (Clinic A), county hospital (Clinic B), and a Veterans Affairs hospital (Clinic C) providing primary, secondary, and tertiary care. Main Outcome Measures: 1) Development of legal blindness in one or both eyes and 2) progression of glaucoma in one or both eyes. Results: Compared to NLB group, LB group features were: higher mean initial intraocular pressure (IOP), (p = 0.03), late detection (p = 0.006), wide variation of IOP in the follow-up period (5.9 vs 4.1 mmHg, p = 0.031), poor control of IOP (p < 0.0001) and non-compliance (p < 0.0003). Conclusions: This study suggests that some of the risk factors for legal blindness in POAG are related to the level of initial IOP, late detection of the disease, poor control of IOP and non-compliance.