Sulfur amino acids have been implicated in the pathogenesis of thromboembolic vascular disease, and observations of patients with several inborn errors of metabolism have led to the ‘homocysteine theory of arteriosclerosis’. Homocysteine is an intermediate in the transsulfuration pathway and it enters into several other reactions, some of which involve transfer of methyl groups. An abnormally high concentration of homocysteine in the blood causes homocystinuria. Deficiency of cystathionine β-synthase is the most frequent cause of homocystinuria. Patients with this disorder are at risk for early vascular occlusions. Treatment with vitamin B6 of patients who are biochemically responsive to this vitamin reduces the risk of thromboembolism. Clinical or pathologic evidence of early vascular disease has also been provided in patients with homocysteinemia due to deficient (re)methylation of homocysteine to methionine. This may be caused by a deficiency of 5,10-methylenetetrahydrofolate reductase or by a deficient synthesis of cobala