Increased level of homocysteine is the cause of arteriosclerosis, vascular disorders, neurodegenerative diseases and mental disorders. This paper presents current views on hyperhomocysteinemia and its pathogenic role in mental disorders. The homocysteine metabolism is described as well as the influence of various factors on it, and also the pathomechanism of alterations resulting from an increased level of homocysteine. There is also a focus on genetic disorders leading to increased level of homocysteine and its correlation with mental disorders and dementia. The multidirectional influence of hyperhomocysteinemia on cardiovascular, immune and neurotransmitter systems is described as well as a neurotoxic action of homocysteine. Complex correlations between vitamin deficiency and hyperhomocysteinemia with depression, cognitive functions impairment and dementia are also presented. Oral supplementation with B-group vitamins and folic acid can significantly decrease a level of homocysteine in blood serum.