Large Optic Disks Vol. 107, No. 2 147 Such physiologic cupping as defined above was found in 20 of 22 eyes (91%) with large optic disks, but in only eight of 32 eyes (25%) with small disks. When cup vs disk diameter was plotted, the two variables were found to be number of eyes linearly related (Fig. 3), 15 1.6 4.7 18 19 2.0 21 22 23 24 25 26 27 disc diameter (mm) Fig. 1 (Maisel and associates). Disk diameter plotted against number of eyes. Quantitative analysis of patients selected for photographs demonstrates 22 eyes with megalopapilla with average disk diameters greater than or equal to 2.1 mm. mean was equal to 0.62." Because two or more standard deviations above the mean accounted for only 7.5% of their study population, 0.6 or more was accepted as the criterion for a large cup/disk ratio in our study.’ This criterion is in agreement with Syndacker’s work’ in which he concluded that optic disks with physiologic cupping exceeding 0.66 occur so infrequently that it is to be considered pathologic until proven otherwise. Rim area—When rim area was plotted against disk area, it was found that in this study population, rim and disk areas vary directly (Fig. 4). Radiation exposure-—Of the 36 patients, 19 had a history of accidental exposure to external whole-body radiation during atmospheric nuclear testing in 1954. Nine patients from the Rongelap atoll received an estimated 1.75 Gy (175 rad) and the ten patients from theisland of Utirik received approximately 0.14 Gy (14 rad).° Of the 36 patients in this study, 19 (a group different from the 19 above) had acceptable photographs of both eyes: 12 of these 19 had been exposed to radiation, five had not, and in two the data were not available. A comparison of the disk sizes in the 12 exposed and five nonexposed patients demonstrates that the two groups haverelatively the same proportion of patients with large disks (Table 1). Evidence of glaucoma—-Features considered to be consistent with glaucomatous optic disk damage other than cupping, such as notching of the neural rim, disk asymmetry, vertical disk cupping, and increased intraocular pressure were investigated. Examination of the neural rim of the optic disk showed 360 degrees of pink neural tissue without notching in all cases. In the 19 patients for whom data were available on both eyes, two had asymmetric Fig. 2 (Maisel and associates). Example of bilateral megalopapilla. Left, Right disk diameter of 2.3 mm. Right, Left disk diameter of 2.2 mm.