The patient, a 4-week-old female infant with vomiting, anemia, leukopenia, thrombopenia, and failure to thrive had the biochemical abnormalities of ketotic hyperglycinemia: metabolic acidosis (pH 7.2; -HCO3 7 mEq liter); hyperglycinemia (8.2 mg/100 ml); increased serum levels of glutamine, glutamic acid, and the ketogenic amino acids as well as a positive urinaty. Acetest and diphenythydrazine reaction. Gas chromatography of the serum voltile fatty acids (VFA) showed a considerable increase of propionic acip up to 70'; of the total VFA content (normal value 3.2%) and a concomitant decrease of acetic acid to 24% (normal value 88%). The results indicate that the acidosis of ketotic hyperglycinemia is accopanied by increased propionic acidemia and allow some specultation on the metabolic block of the disorder.