In Europe, neuropsychiatric diseases currently make up approximately a third of the total burden of disease. In 2004, 27% of the overall population was affected by at least one of the most frequent neuropsychiatric diseases such as Alzheimer's dementia, Parkinson's disease, stroke or depression. The annual costs of care exceed those of cancer, cardiovascular conditions and diabetes. In order to delay the onset or course of neurodegenerative diseases, the available potential should be utilized. As well as improving quality of life of patients and relatives, this may reduce the great financial burden caused by neurodegenerative disorders. However, the availability of established drugs or therapeutic agents is very limited. This paper reviews the state of current knowledge as to how homocysteine metabolism is relevant for neurodegenerative and other neuropsychiatric diseases, with particular emphasis on the evidence for prophylactic and therapeutic strategies. In the European countries, many people do not take the recommended daily minimum amount of folate and vitamin B12. Deficiency of these vitamins and secondary changes in the concentrations of associated metabolites, such as methylmalonic acid and homocysteine, may contribute to the onset and progression of neuropsychiatric diseases. This paper reviews the evidence regarding whether substitution of folate and vitamin B12 is beneficial, for example, in cerebrovascular disease, dementia and depression.