Homocystinuria is a genetically determined inborn error of the methionine amino acid pathway characterized by increased plasma homocysteine. In its major form, homocystinuria, is due to cystathionine beta synthase deficiency. Treatment of these adulthood patients lead physicians to call up on the skilled advices of pediatricians. But prevention and treatment of age related vascular and osteoporotic complications are still to be evaluated.