In Finland, 19 children, born 1964--1977, from 13 families, have been diagnosed as suffering from nonketotic hyperglycinemia (NKH). This gives an incidence for NKH in the Finnish population of 1:55,000 newborns. The majority of these children were born in the northern part of the country, where the incidence is 1:12,000. The geographical distribution of the birth-places of the grandparents also seems to point towards an enrichment of the gene in northern Finland. An autosomal recessive mode of inheritance for this disease seems probable, since the corrected proportion of affected siblings (Apert's a priori method) is 0.288. Abnormally high plasma glycine concentration and elevated glycine urinary excretion in the parents of the NKH-children suggest the existence of a minor metabolic defect in heterozygotes of this disease. Some of the healthy siblings of the NKH-patients also show similary elevated levels. However, a definite diagnosis of the NKH-heterozygote state cannot easily be made on the basis of these laboratory findings, as the levels in some individuals are very close to, or even overlap corresponding values in a normal material.