A 15-year-old girl with a former clinical diagnosis of cerebral palsy was found to have isolated deficiency of 3-methylcrotonyl-CoA carboxylase (MCC) on gas chromatography-mass spectrometry (GC/MS) analysis and enzyme determination. Her symptoms included marked growth retardation from birth, profound mental retardation, tonic seizures, rigospastic quadriplegia with opisthotonic dystonia, gastroesophageal reflux with poor esophageal peristalsis, and recurrent episodes of aspiration pneumonia. Brain MRI revealed marked brain atrophy, involving both the gray and white matter. Although she did not exhibit acute metabolic decompensation or acute encephalopathy, her neurological symptoms continuously worsened. This patient is the oldest among reported cases of MCC deficiency who had symptoms at birth, and this case may have the severest sequelae of the longest known natural course of this inborn error of metabolism.