Using planimetric analysis of stereoscopic optic disk photographs, we studied 21 optic nerve heads of 11 subjects who shared the common feature of optic cups that were larger than the mean + 2 S.D. within the normal population. A comparison of these findings to those of 571 normal optic disks and 706 optic nerve heads in eyes with chronic primary open-angle glaucoma showed the following morphologic characteristics: (1) abnormally large optic disk area (mean +/- S.D., 4.49 +/- 0.56 mm2), (2) large cup/disk ratios with the horizontal ratio (0.78 +/- 0.03) significantly (P less than .001) larger that the vertical (0.71 +/- 0.03), (3) increased incidence of cilioretinal arteries, (4) normal neuroretinal rim area (2.06 +/- 0.35 mm2), (5) normal neuroretinal rim configuration, inferiorly (0.43 +/- 0.08 mm) broader (P less than .001, Wilcoxon test) than superiorly (0.33 +/- 0.06 mm), smallest (P less than .0001) temporally (0.20 +/- 0.04 mm), (6) normal form of zone alpha (irregular hypopigmentation and hyperpigmentation) of the parapapillary chorioretinal atrophy with its widest extension in the temporal horizontal area, (7) no zone beta (visible large choroidal vessels and sclera), (8) normal caliber of the parapapillary retinal vessels, and (9) normal parapillary retinal nerve fiber layer. These characteristics are helpful in the differentiation of primary and secondary large cups.