We studied optic disc photographs of 35 eyes to determine the level of agreement between estimates of vertical and horizontal cup-to-disc ratios (CDRs) provided by two clinicians and estimates of these ratios obtained using an image analyzer. We also evaluated the agreement of each clinician with himself and with the other clinician to provide a relative standard by which to judge agreement between the clinicians and the image analyzer. Agreement between the clinicians and the image analyzer was moderate. Differences of more than 0.2 disc diameters (DD) between the CDR estimates of clinicians and those obtained using the image analyzer were limited to discs having small (CDR of 0.4 DD or lower) or very large (CDR of 0.9 DD or higher) cups. The clinicians' agreement with themselves and with each other was substantial to near perfect.