Primary open-angle glaucoma (POAG), by far the most common form of glaucoma, currently afflicts more than 2 million Americans, more than half of whom probably don't know they have it. Asymptomatic in the early stages, it gradually and progressively reduces the visual field and leads to blindness if untreated. Elevated intraocular pressure is believed to play a role, yet some people with normal pressure develop POAG. There is no single diagnostic test; screening and diagnosis involve periodic comprehensive eye evaluations. Treatment is aimed at delaying onset, slowing progression, and preserving vision.