Excessive homocysteine levels in blood. It is often associated with folate or cobalamin deficiency as well as genetic defects. Severity of symptoms is determined by how high the homocysteine levels are. Sufferers are generally asymptomatic until the onset of premature arterial disease later in life. Other symptoms such as mental retardation only occur in severe cases where the homocysteine levels are extremely high. See also Hyperhomocysteinemia