Vitamin B12 and folate concentrations in serum and cerebrospinal fluid of neurological patients with special reference to multiple sclerosis and dementia. Vitamin B12 and folate concentrations were measured in serum and cerebrospinal fluid (CSF) in 293 neurological patients. Serum and CSF vitamin B12 concentrations showed a positive correlation. In individual patients CSF B12 concentrations varied considerably for a given serum concentration. The median serum vitamin B12 concentration of the Alzheimer's type dementia group was significantly lower compared with that of a control group. Lower median CSF vitamin B12 concentrations were found in groups of patients with multiple sclerosis and Alzheimer's type dementia. Five patients with heterogeneous clinical pictures had unexplained low serum and CSF B12 concentrations without macrocytosis. Two patients had very high serum B12 and low-normal CSF concentrations which could be explained by a blood-brain barrier transport defect. Serum and CSF folate concentrations did not show significant differences between the various groups. 