Anatomic evidence indicates that glaucoma initially destroys large axons before affecting smaller ones. Large axonal size has been correlated with fast, transiently responding retinal ganglion cells (alpha/Y cells). A second population of cells (X cells) does not follow flicker stimulation to as high a frequency as Y cells. The luminance flicker visual-evoked potential (VEP) shows two response maxima as a function of temporal frequency. These two response peaks may indicate driving of the flicker VEP by two separate neuronal populations, possibly X and Y cells. The authors have recorded flicker VEPs as a function of frequency from both normal subjects and glaucoma patients with asymmetric visual field loss in various stages of their disease. The data obtained demonstrate loss of high flicker rate responses apparently preceding and correlated with perimetric field loss and stage of glaucoma. Flicker VEP responses below 13 Hz tend to be preserved, whereas above this frequency the response attenuation is directly proportional to the severity of visual loss. This technique may provide an early warning of ganglion cell loss in the early stages of this disease.