The pattern electroretinogram (PERG), a noninvasive tool to measure the activity of retinal ganglion cells, shows changes in early glaucoma. We wanted to investigate whether or not the PERG is preferentially affected when the Bjerrum area is stimulated. We recorded the PERG from 17 eyes in 17 patients with initial stages of visual field defects due to open-angle glaucoma and from 10 eyes in 10 normal subjects. Counter-phasing checkerboard patterns were used as visual stimuli with a check size of 0.8 degrees at 16 reversals/s and 98% contrast. Three visual field extents were stimulated: (1) full screen (31 degrees x 27 degrees), (2) central stimulation with a diameter of 14 degrees, (3) peripheral stimulation = full screen with central mask of 14 degrees in diameter. The latter stimulus covers the regions where traditionally glaucomatous defects are expected early. In glaucoma patients, there was a significant decrease in the PERG amplitude compared to normals. This reduction was very similar for all three stimuli (42%, 52% and 47%, respectively). Thus, the PERG amplitude is reduced to a similar degree both in regions of the visual field where early damage is found (Bjerrum area) and in the central area. This result suggests that in early glaucoma there is focal as well as diffuse retinal damage.