Serum vitamin B12 levels (as determined by radio-immunoassay) were measured in 20 subjects aged 65 years and over with Alzheimer-type dementia, 20 age-matched subjects with non-Alzheimer type dementia and 20 age-matched subjects with no dementia. Serum vitamin B12 levels were significantly lower and serum vitamin B12 deficiency was significantly more frequent in subjects with Alzheimer-type dementia and were independent of age, sex, haematological abnormality or serum folate.