A new inborn error of metabolism was found in two mentally retarded children. The excretion of homocystine was accompanied in one boy by ectopia lentis, failure to thrive, poor developmental milestones, thrombo embolic phenomena, methioninuria, decreased excretion of taurine, and elevated plasma methionine. The second patient had normal plasma methionine levels, and a nearly normal amino acid pattern in the urine other than homocystine. Loading tests using methionine, serine, and homocystine failed to alter plasma amino acid levels and did not provide any information on the nature of the metabolic defect in sulfur amino acid metabolism. The two children had different abnormalities in methionine metabolism or they had the same metabolic defect but in different degrees of severity.