PURPOSE: To evaluate the use of disk diameter as an indicator in the identification of glaucomatous optic neuropathy. METHODS: We evaluated all available stereofundus photographs for 1,040 right eyes obtained in the Reykjavik Eye Study. Horizontal and vertical disk diameters were determined in a masked manner by a glaucoma specialist (K.F.D.). All disk diameters were corrected for refractive error. RESULTS: There were significant differences (P < .05) between the corrected vertical disk diameters of normal subjects (0.189 ± 0.018 inches) and those suspected of having glaucoma (0.202 ± 0.020 inches) as well as between the normal and the glaucoma groups (0.206 ± 0.029 inches). The corrected horizontal measurement showed the same pattern. CONCLUSIONS: In the Reykjavik Eye Study, optic disks meeting structural criteria for glaucoma are significantly larger than normal nerves.