The performance of patients with multiple sclerosis on selected psychological tests was examined to ascertain the usefulness of such examinations to diagnosis. Cognitive impairment was studied in relationship to disease-related factors, physician's identification of cerebral involvement, and psychological adjustment. The results indicate that half the subjects exhibited cognitive impairment. Levels of neurologic involvement, physical impairment, and depression were not predictive of cognitive impairment. Of the subjects who were judged on neurological examination to have intact mentation, half were actually impaired. Impaired cognitive functioning, which is often not detected through routine examination, may occur early in the disease. These deficits may represent manifestations of otherwise undetectable plaques in the subcortical white matter.