The concentration of homocysteine, a sulfur-containing amino acid, in the bloodstream, where elevated levels mark increased cardiovascular risk. Homocysteine forms during methionine metabolism and is normally recycled with the help of folate, vitamin B12, and vitamin B6; when these B vitamins run short, homocysteine accumulates. High levels damage blood vessel linings and promote clotting, associating with heart disease, stroke, and cognitive decline in observational studies. B-vitamin supplementation reliably lowers it, though trials show this lowers events less dramatically than hoped, suggesting it is partly a marker rather than purely a cause. Measuring it helps identify B-vitamin deficiency and metabolic disturbances, making it a useful, if imperfect, window into vascular risk.