Prenatal diagnosis of inborn errors of amino acid metabolism was discussed from the viewpoints of its purpose, ethical problems, analysis methods, and sampling methods of the fetal tissues. In addition to the general discussion, our results of the prenatal diagnosis of 20 cases with nonketotic hyperglycinemia (NKH) were also reported. Nineteen cases were diagnosed by the enzymatic method, while one Finnish case was successfully diagnosed by DNA analysis, which is based on our observation that a missense mutation we identified accounts for 70% of the mutant alleles in Finland. The DNA analysis would be a great help for the prenatal diagnosis in Finland where the incidence of NKH is unusually high (1: 12,000 births).