• Optic neuropathy in multiple sclerosis is often not detectable by conventional clinical assessment. Temporal resolution of vision was measured at localized retinal sites by a simple perceptual test of double light-flash discrimination. Nine of 11 multiple sclerosis patients with normal fundi and no prior history of optic neuritis displayed abnormal temporal resolution. Double-flash discrimination was consistently more sensitive than any of the clinical variables measured and in some cases complementary to them. The more advanced the disease, the more spatially widespread was the double-flash impairment in the central visual field. There was a significantly increased level of double-flash impairment in advanced multiple sclerosis patients compared with patients with spinal forms of the disease and with unilateral retrobulbar neuritis.