Homocystinuria is an autosomal recessively inherited defect of methionine metabolism (the transsulfuration or methylation pathway), leading to an abnormal extracellular accumulation of homocysteine and its metabolites.

The diagnosis is usually made when subluxation of the ocular lens occurs.

Affected individuals with homocystinuria manifest skeletal abnormalities resembling those of Marfan syndrome. Thromboembolic episodes involving both large and small vessels, especially those of the brain, are common and may occur at any age. Progressive mental retardation is common.