In recent years, the role of Vitamin B12 deficiency in the etiopathogenesis of multiple sclerosis has been debated. Because of this we determined serum Vitamin B12 and folate -due to close relation with Vitamin B12- levels as well as mean corpuscular volume of multiple sclerosis patients. We compared patients with healthy controls and investigated correlation of the parameters with the age of the patient, the age at the beginning of the disease and the duration of the illness. We examined 24 multiple sclerosis patients at acute attack but before treatment and 20 age-matched healthy people. The mean Vitamin B12 level is lower in patient group significantly ( 560± 295 pg/ml in patients, 1091± 752 pg/ml in controls; p<.01 ). Mean folate levels and mean corpuscular volumes does not differ. The age of the patient, the age at the beginning of the disease and the duration of the illness does not correlate significantly with the parameters investigated. Although it does not cause the disease itself, the low level of Vitamin B12 may render some patients more vulnerable to the viral and immunologic processes which are suspected for the etiology of multiple sclerosis.