Malonyl-CoA decarboxylase (MCD) deficiency is a rare inborn error of metabolism that affects beta-oxidation of fatty acids. At this time, over 20 patients with this condition have been reported. Symptoms of MCD deficiency include developmental delay, hypertrophic cardiomyopathy, seizures, acidosis, hypogylycemia and hypotonia as the most common features reported. Patients with no or only mild developmental delay have been described as has a patient with significant structural brain abnormalities. Identification of MCD deficiency may increase as a result of tandem mass spectrometry-based newborn screening.