Studies of normal subjects and of patients with optic nerve disorders suggest that one of the components of the pattern electroretinogram (PERG), the second negative wave, is related to optic nerve function and appears to be diminished even when the condition is relatively mild, with little alteration of visual acuity. It is known that significant loss of nerve fibers may occur prior to the development of visual field loss. We have investigated the PERGs of a group of subjects with early glaucoma and ocular hypertension, comparing them with normal subjects, and have found a selective reduction in the second negative wave in patients with evidence of optic nerve dysfunction. The PERG may prove helpful in discriminating those patients with ocular hypertension who are destined to develop visual field loss unless medical or surgical therapy were to be employed.