Isolated 3-Methylcrotonyl-CoA Carboxylase (3-MCC) deficiency is caused by defects in the mitochondrial 3-MCC enzyme. The phenotype of 3-MCC deficiency is highly variable ranging from severe neurological abnormalities and death in infancy to asymptomatic adults. A severe presentation of 3-MCC deficiency may include a Reye-like illness, ketoacidosis, hypoglycemia, hyperammonemia, psychomotor retardation, seizures, symptoms of cardiorespiratory failure and coma, while a mild presentation can include fatigue and weakness during catabolic episodes or mild developmental delay. Presentations with cardiomyopathy, brain atrophy and fatty infiltration of liver or muscle may also occur. This disorder is the most frequent organic aciduria detected in tandem mass spectrometry-based newborn screening programs. Often, a child with a positive newborn screen will have follow-up testing consistent with 3-MCC deficiency but never present with symptoms of the disorder. A positive newborn screen in an infant may also lead to the detection of an asymptomatic mother and siblings.