Elevated serum homocysteine, decreased folate and low vitamin B(12) serum levels are associated with poor cognitive function, cognitive decline and dementia. Despite evidence of an epidemiological association, randomised controlled trials did not provide any clear evidence so far that supplementation with vitamin B(12) and/or folate improves dementia or slows cognitive decline, even though it might normalise homocysteine levels. In this report, we review the current knowledge on the relationship between homocysteine, folate and vitamin B(12) levels and the way their disruption influences cognitive function in adults.