SummaryA program of newborn mass screening for inborn errors of metabolism has since 1977 been conducted by The Ministry of Health and Welfare in Japan with great success in preventing mental and physical handicaps in children and estimating the incidence of these diseases in our country as follows: phenylketonuria (PKU) 1/78,100 maple syrup urine disease (MSUD) 1/281,200 histidinemia 1/10,600 galactosemia 1/156,200There are some genetic variants of each of these diseases, PKU, MSUD and others. Circumspection is therefore required in making a diagnosis or selecting a particular method of treatment. When MSUD is suspected, it is necessary to make a definite diagnosis promptly and take therapeutic measures including peritoneal dialysis.The efficacy of low histidine diet in histidinemia is not yet ascertained. Close inquiries should be made about the benefit of such a dietary regimen and the long term prognosis therewith. A screening test for homocystinuria by the detection of blood methionine level is difficult. In cases with high blood methionine level during the neonatal period, it is necessary to differentiate the condition from other disease with high blood methionine.Hepatorenal tyrosinemia appears to be unsuitable for mass screening because of the the unefficacy of low phenylalanine tyrosine diet in some of the patient and the difficulty to differentiate from transient tyrosinemia in normal newborn infants. However, mass‐screening is recommendable for Richner‐Hanhalt type tyrosinemia which responds well to dietary therapy and which is easy to differentiate from the transient tyrosinemia because of the marked elevation of blood tyros