A possible approach to evaluation of mild cognitive deficits in patients with multiple sclerosis (MS) is provided in the article. The results of the approbation of authors' experimental technique for studying cross-modal interactions that was carried out in healthy adolescents and MS patients are presented. Perspectives of its using in further research and some preliminary data on deficits in cross-modal syntheses in adolescents with MS are discussed. The role of impairment of interactions between different cognitive functions in the development of cognitive deficit is emphasized.