Non-ketotic hyperglycinemia (NKH) has been differentiated as an autosomal recessive hereditary form of the hyperglycinemias with a defect in the glycine-cleavage system causing accumulation of glycine in all body fluids. A more severe neonatal form with early onset has been described and, though not curable, a fast and correct diagnosis for clinical management, parental information and genetic counselling is important. The clinical picture and diagnostic chemical analyses in two cases of the more severe neonatal form of NKH are reported.