One consequence of vitamin B12 deficiency is a raised blood level of homocysteine. This non-essential amino acid is derived from dietary protein. Its conversion to useful metabolites requires methyl-folate and vitamins B12 and B6 as cofactors. Blood levels of homocysteine therefore rise with deficiency of these important vitamins. Andrew McCaddon, Honorary Senior Research Fellow in the School of Medicine at Cardiff University, provides a case study of his finding of low B12 levels in families with early onset dementia and his search for evidence of deficiency in patients with late onset dementia.