To determine cognitive disturbances in recent demyelinating disease, we studied 21 patients with definite or probable multiple sclerosis (MS) of less than two years' duration and nine patients with recently isolated optic neuritis. None had any clinical or social evidence of cognitive impairment. Mild to moderate cognitive impairment. Mild to moderate cognitive impairment was present in 18 (60%) of 30 cases, affecting visual and/or verbal efficiency. These abnormalities were statistically significant when compared with the results of a control group of 29 patients. There was no correlation with a depressive status, between the presence of cognitive impairment and either the degree of handicap or the activity of the disease. The frequency of cognitive dysfunction (60%) appears to be comparable to that reported in other series in which MS evolution is over ten years. The natural history of cognitive functions in MS has to be identified. Neuropsychologic tests could be useful in the diagnosis of monosymptomatic or paucisymptomatic forms of MS (ie, visual or medullary).