Vitamin B12 levels in the serum and the cerebrospinal fluid (CSF) were compared between patients with Alzheimer's disease (AD) and senile dementia of Alzheimer's type (SDAT) (AD group) and patients with multi-infarct dementia (MID group). The B12 levels in the serum and the CSF were 742 +/- 359 pg/ml and 28 +/- 7 pg/ml (mean +/- SD), respectively, in the AD group, and 962 +/- 254 pg/ml and 50 +/- 26 pg/ml, respectively, in the MID group. CSF B12 levels were significantly lower in the AD group than in the MID group, whereas the serum levels were not different. At the same time, the serum levels of almost all patients were within the normal range, whereas the CSF levels were 25 pg/ml or lower in 10 of 12 AD patients. Therefore, this low level in the CSF is considered to be a characteristic finding in the AD group.