Vitamin B12 and to a lesser extent folate deficiencies have been associated with dementias. Both these vitamins are determinants of plasma total homocysteine concentrations. In this review the frequency distributions of plasma vitamin B12, folate and homocysteine in South African males (# 51 yrs and > 51 yrs) illustrate the lower vitamin B12 levels in older subjects, and the shift toward elevated homocysteine concentrations in elderly people. Vitamin B12 deficiency appears to be associated with neuropsychiatric disorders, including dementias, but no causal relationship based on biochemical evidence has so far been established. Supplementation with vitamin B12 improves some neurological abnormalities and reverses only mild dementia of recent onset, but does not slow the progression of dementia. Elevated homocysteine levels appears to affect cognitive function, as measured by spatial copying skills and visual event-related potentials. Measurement of plasma homocysteine may help identify individuals with vitamin deficiencies and hyperhomocysteinemia. The relation between B-vitamins, homocysteine and dementia needs to explored further before vitamin supplementation is advocated to prevent or reverse neuropsychiatric disorders.