The purpose of this study was to determine how closely peripapillary retinal vessel diameter is related to functional and structural optic nerve damage in primary open-angle glaucoma. Using optic disc photographs of 234 eyes of 141 patients with primary open-angle glaucoma and 139 eyes of 86 normal subjects, the diameters of the superior and inferior temporal retinal arteries and veins were measured at the optic disc border. On the basis of rim/disc area ratio, the glaucoma group was divided into four stages: early, more than 0.61; medium, 0.60~0.41; advanced, 0.40~0.21; far advanced, less than 0.20. In the normal group the diameter of the inferior temporal vein was the largest, followed by that of the superior temporal vein, the inferior temporal artery, and the superior temporal artery. The diameters of the inferior and superior temporal retinal artery were significantly smaller at the early and medium stage, respectively, whereas both inferior and superior temporal retinal vein diameters were significantly smaller at the far advanced stage. The diameters of the inferior and superior temporal retinal arteries correlated significantly with neuroretinal rim area (r > or = 0.48, P = 0.0001), mean deviation (r > or = 0.42, P = 0.0001), vertical cup-to-disc ratio (r < or = -0.33, P = 0.0001), and peripapillary atrophy data (r < or = -0.14, P < 0.04). The results indicate that in primary open-angle glaucoma, vessel diameter becomes less as neuroretinal rim area decreases and visual field defects and peripapillary atrophy increase. Its evaluation can be helpful for the diagnosis of glaucoma and possibly also during follow-up.