Cerebrospinal fluid (C.S.F.) findings of 68 patients in multiple sclerosis in relation to the length of illness are reported. Immune reactions in the C.S.F. are similar to those reported by other research workers. In the first 10 years illness an increasing blood-C.S.F. barrier impairment can be noticed. Similar C.S.F.-diagnoses, immune reaction and barrier disturbance, are also possible in acute and severe stages of multiple sclerosis. Consequently, barrier impariment during a severe reaction in the C.S.F. in multiple sclerosis can be an indicator of an increase of pathomorphological C.N.S.-changes which is possible in the first 10 years of illness.