Background Associations between low levels of folate and vitamin B 12 and cognitive impairment in patients with dementia have been reported. Some studies revealed correlations between low levels of vitamin B 12 and behavioural and psychological signs and symptoms of dementia (BPSD) in Alzheimer's disease (AD) patients. Given the lack of studies in frontotemporal dementia (FTD) and on folate and given the methodological shortcomings of former publications, we set up a prospective study. Methods At inclusion, AD (n ¼ 152) and FTD (n ¼ 28) patients underwent a neuropsychological examination. Behaviour was assessed using a battery of behavioural assessment scales. Determination of serum vitamin B 12 and red cell folate levels were performed within a time frame of two weeks of inclusion. Results In both patient groups, significantly negative correlations between levels of serum vitamin B 12 and red cell folate and the degree of cognitive deterioration were found. No correlations with BPSD were found in the AD patient group. In FTD patients, levels of vitamin B 12 were negatively correlated with both hallucinations ( p ¼ 0.022) and diurnal rhythm disturbances ( p ¼ 0.036). Conclusions The observed negative correlations between levels of vitamin B 12 and folate and cognitive impairment in both AD and FTD patients, raise the possibility of a non-specific etiological role. Although levels of vitamin B 12 and folate did not correlate with BPSD in AD patients, negative correlations between serum vitamin B 12 levels and BPSD in FTD patients were revealed. Decreased serum vitamin B 12 levels may predispose FTD patients to develop hallucinations and diurnal rhythm disturbances.