Objective: This study aimed to determine the probability of a patient developing legal blindness in either one or both eyes from newly diagnosed and treated open-angle glaucoma (OAG) after starting medical or surgical therapy or both. Design: The study design was a retrospective, community- based descriptive study. Participants: Two hundred ninety-five residents of Olmsted County, Minnesota, newly diagnosed with, and treated for, OAG between 1965 and 1980 with a mean follow-up of 15 years (standard deviation ± 8 years) participated. Intervention: Kaplan-Meier cumulative probability of blindness was estimated for patients treated and followed for OAG. Main Outcome Measures: Legal blindness, defined as a corrected visual acuity of 20/200 or worse, and/or visual field constricted to 20°or less in its widest diameter with the Goldmann III4e test object or its equivalent on automated perimetry, secondary to glaucomatous loss, was measured. Results: At 20- years' follow-up, the Kaplan-Meier cumulative probability of glaucoma-related blindness in at least one eye was estimated to be 27% (95% confidence interval, 20%-33%), and for both eyes, it was estimated to be 9% (95% confidence interval, 5%-14%). At the time of diagnosis, 15 patients were blind in at least 1 eye from OAG. Conclusion: A retrospective study of a white population determined that the risk of blindness from newly diagnosed and treated OAG may be considerable.