Folate and vitamin B12 deficiency manifests in a number of known hematological and neurological disorders. Recently, an association has been discovered between these nutrients and Alzheimer's disease (AD). Although the initial evidence suggested that vitamin B12 levels may be reduced in AD patients, a number of studies failed to establish a consistent association between global deficiency in B12 and folate with this disease. However, more careful analysis of the biochemistry of these vitamins has revealed consistent metabolic abnormalities in Alzheimer's patients. The presence of elevated methylmalonyl-CoA and homocysteine levels in AD patients has suggested a number of interesting mechanisms by which they may play a role in the pathogenesis of dementia. There is currently a lack of consensus, however, as to whether these metabolic abnormalities represent an etiological factor or simply a consequence of the disease process.