We measured relative afferent pupillary defects in 105 patients with various stages of optic neuritis. We detected pupillary defects in 96% of acute unilateral cases, 92% of recovered unilateral cases 91.7% of acute cases with evidence of optic neuropathy in the other eye, and 65.8% of recovered bilateral cases. With careful testing, one can find relative afferent pupillary defects in nearly all patients with unilateral optic neuritis and in most patients with bilateral disease. A patient without a pupillary defect after apparent unilateral optic neuritis frequently has evidence of disease in the other eye when visual-evoked potentials are tested.