In addition to neurological symptoms, multiple sclerosis is characterized by cognitive function impairment. Disturbances of memory, recall, information processing, visual-spacial perception, attention, and executive function, in less extent of speech, are present in about 60% of patients. They are similar to disorders in other subcortical dementias. Once they appear, they rarely recede. Conventional, and especially nonconventional magnetic resonance imaging evaluates more precisely the tissue substrate--diffuse neuroaxonal lesion of the entire brain parenchyma--than clinical findings, already in the early stage of the disease. Alterations in the brain imaging are manifested by T2 hyperintensive and T1 hypointensive lesions, decreased neuronal marker N-acetyl-aspartate in magnetic spectroscopy, decreased magnetization transfer ratio, and increased diffusivity with reduced anisotropy in diffusion-weighted imaging. Total volume of brain lesion, corpus callosum diameter, and relation of measures of brain chambers and the rest of the brain, are best indices of cognitive dysfunction in multiple sclerosis. Their diagnosis in the very beginning of the disease allows early application of therapeutic procedures. Symptomatic treatment of these disorders is not efficient, and immunomodulation, particularly the use of biologic versions of interferon-beta, shows disputable effects. Cognitive dysfunctions affect relationships and working ability of patients.