We studied the relationship between intraocular pressure (IOP) and progression of visual filed loss in 83 eyes of 83 patients with advanced primary open-angle glaucoma (loss of at least one quarter of the visual field on Goldmann perimetry). During the 15-year follow-up study, 71 eyes (86%) showed progression of visual field loss. The mean IOP was significantly lower in eyes that remained stable (13.4 +/- 1.3 mm Hg) than in those that showed progression of visual field loss (19.4 +/- 2.6 mm Hg). These results suggest that it is necessary to reduce IOP to below 15 mm Hg to prevent further progression of visual filed loss in advanced glaucoma.