Homocystinuria: A genetic disease that is due to an enzyme deficiency that permits a buildup of the amino acid homocysteine. Progressive mental retardation is common, but does not always occur, in untreated cases of homocystinuria. The finding of vascular disease and premature arteriosclerosis in persons with homocystinuria led to the theory that homocysteine may be a factor in heart disease. Homocystinuria is inherited in an autosomal recessive manner and is one of the diseases commonly included among the diseases for which newborns are screened. Treatments include special diets and vitamin B6.