We measured central contrast sensitivity in both eyes of 27 patients with asymmetric glaucomatous visual field loss or optic disc cupping. In 15 patients contrast sensitivity was less in the eye that by perimetry or ophthalmoscopy was the more severely damaged. In 10 patients contrast sensitivity was the same in the two eyes. In two patients, it was impaired more in the eye with the normal visual field. However, the latter two patients had ophthalmoscopic evidence of optic nerve damage (disc hemorrhage or large cup) in the eye with the lower contrast sensitivity, even though the visual field was normal. Asymmetry of contrast sensitivity was not found in normal control subjects. These results suggest that glaucoma does alter central vision, even when visual acuity remains normal and visual field defects are far from fixation. Contrast sensitivity may be impaired by a different mechanism than that which leads to visual dield loss. Although the two types of visual dysfunction often occur together, some patients may have more severe impairment of central vision (as measured by contrast sensitivity testing) in one eye and more severe loss of peripheral vision (as measured by perimetry) in the other eye.