3-Hydroxy-3-methylglutaryl-coenzyme A lyase deficiency (HMG-CoA lyase) is an inborn error of leucine catabolism which often leads to life-threatening illness in the neonatal period. The cardinal clinical features include severe infantile hypoglycemia, metabolic acidosis, hepatomegaly, lethargy or coma and apnea. Hyperammonemia is variable. There is a characteristic absence of ketosis. Considerable heterogeneity has been observed in clinical and biochemical presentation. Acute episodes of illness have been mistaken for Reye syndrome. The pattern of organic acids in the urine includes large amounts of 3-hydroxy-3-methylglutaric, 3-methyl-glutaconic, 3-methylglutaric and 3-hydroxyisovaleric acids. Smaller, but appreciable levels of glutaric, adipic and other dicarboxylic acids may also be excreted in the urine. Lactic acid may be present in sizable amounts at times of acute illness. The primary defect is a deficiency of 3-hydroxy-3-methylglutaryl-coenzyme A lyase, a key enzyme in the cycle of ketogenesis.