The authors prospectively analyzed T-lymphocyte subpopulations in the peripheral blood of nine patients with acute unilateral optic neuritis and compared them with 25 controls without neurologic disease. The presence or absence of alterations in circulating T-cell subsets has not been examined previously in patients with isolated optic neuritis. The authors found the mean ratio of inducer (CD4) to suppressor (CD8) T-lymphocytes was 2.07 +/- 0.51 for the group with optic neuritis, statistically indistinguishable from a value of 1.78 +/- 1.04 for the control group. Multiple sclerosis (MS) subsequently developed in one patient. Her inducer/suppressor T-cell ratio was initially 2.66, but progressively increased to 3.68 concomitant with the clinical manifestation of focal neurologic signs. Although optic neuritis may be the initial clinical sign of MS, the periodic alteration of circulating T-lymphocytes increasing the inducer/suppressor T-cell ratio in MS was not observed in those with isolated optic neuritis.