A newborn mass-screening program for the early detection of phenylketonuria, maple syrup urine disease, homocystinuria, galactosemia, congenital hypothyroidism, congenital adrenal hyperplasia, using filter paper blood specimens, was started throughout Japan in 1977. The total number of newborns screened by March 2000 reached 29,657,738; this represents 95% of the newborns during this period. A collaborative study group has performed a follow-up study of the cases detected by this program since the start of this screening program. The results we have obtained through this study to date include: hyperphenylalaninemia, 1:70,000; congenital hypothyroidism, 1:5,000; and, congenital adrenal hyperplasia, 1:20,000. The cases of maple syrup urine disease, homocystinuria, and galactosemia type 1 were too few for a reliable incidence. Accumulated data for PKU show that IQ is inversely related to blood phenylalanine level and stricter dietary control guidelines have resulted. We now have a number of adolescents with PKU and long-term follow-up data are being obtained.