Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is the most common disorder of fatty acid oxidation. Newborn screening by tandem mass spectrometry reveals the accumulation of octanoylcarnitine, which is characteristic of MCAD deficiency. Symptoms that typically occur between 6 months to two years include lethargy, hypoglycemia, vomiting, hypotonia, seizures and sudden infant death syndrome. Complications include hepatic dysfunction, respiratory difficulties, cardiac arrest, neurologic deficits and coma. The disorder is characterized by high mortality however milder variants exist and adult onset can occur. Significant phenotypic heterogeneity may occur even within a family.