A 5-month-old infant had an unusual combination of clinical signs and symptoms. These consisted of irritability, dystonia, lack of head control, grimacing, opisthotonos, choreoathetoid movements, delayed development, and severe metabolic acidosis. Metabolic investigation by gas-liquid chromatography/mass spectrometry detected urinary organic acids. This confirmed the diagnosis of L-glutaric aciduria. The concentration of L-glutaric acid in the patient's plasma was 2.5 mg/dl (normal range, 0 to 0.1 mg/dl), and in the patient's urine was 4.6 mg/mg of creatinine (normal range, 0 to 0.05 mg/mg of creatinine), but the concentration was not elevated in the plasma and urine of the infant's parents nor of two other family members. No glutaryl-CoA dehydrogenase activity was found in leukocytes taken from the patient. Three of the four family members, including the parents, demonstrated 38%, 42%, and 42% activity, respectively, compared with the activity of normal controls. These findings are consistent with an autosomal recessive disorder involving the metabolism of glutaryl-CoA to crotonyl-Co-a. Dietary restriction was instituted on two separate occasions. First, a low protein diet of 1.6 gm/kg of body weight per day was given, then a low lysine intake of 50 mg/kg/day. These dietary manipulations caused a decrease in the plasma and urine concentrations of L-glutaric acid and beta-hydroxyglutaric acid. However, no effect on the clinical manifestations of the disease was noted.