The authors studied 232 glaucoma patients in an attempt to shed light on the implications of the pit-like localized cupping of the optic nerve often referred to as an "acquired pit of the optic nerve" (APON) both in low-tension glaucoma and in glaucoma associated with elevated intraocular pressure (IOP). Twenty (74%) of the 27 patients with low-tension glaucoma had APONs, whereas 31 (15%) of the 232 patients with elevated pressure did (P less than 0.001). Overall, there was no difference between the degree of field loss in patients with APONs and in those without. We suggest that APONs may signal an abnormal susceptibility of the optic nerve to the damaging effects of IOP.