The similarity in symptomatology of acquired cone dysfunction syndrome and certain optic nerve diseases led to a comparison of these disorders. Without prolonged follow-up, clinical differentiation was usually quite difficult. Aberrations in retinal function tests that have been considered typical of cone dysfunction were also found in these optic neuropathies. It was concluded that abnormalities in these tests were not specific for the cones and indicated a disturbance anywhere within the photopic visual system. However, patients with cone dysfunction regularly demonstrated improvement in acuity with reduction in illumination in contrast to normals and patients with optic nerve or macular disease. This response was found to be a clinically significant factor.