Hyperhomocysteinemia is generally acknowledged as a treatable risk factor for atherotrombotic diseases, but a causal relationship between both is not yet definitively established. Hyperhomocysteinemia originates from a deviation in the methionine-homocysteine metabolism including disturbances of enzymes, vitamin deficiencies and different other factors. Observational studies, genetic polymorphism studies and several meta-analyses implicate already a causal relation between homocysteine and cerebrovascular diseases. It is useful to determine homocysteine levels for stroke who present no clue for vascular disease and thrombosis, who have an ischemic stroke at a young age and who have a family history of premature atherosclerosis. Because of the low cost and safety of the therapy, the American Heart and Stroke Association advises to treat patients with a stroke and hyperhomocysteinemia daily with 0,4 mg folic acid, 2,4 microg vitamin B12 and 1,7 mg vitamin B6. A significant benefit in secondary prevention is not yet proven. The results of larger follow-up trials have to be published.