Reversible clinical and magnetic resonance imaging findings in late-onset cobalamin C defect: Cobalamin C (Cbl C) disease is an inborn error of intracellular cobalamin metabolism. Two distinct clinical types are defined according to the age of onset. We describe an 8 year old girl with late-onset Cbl C disease presenting with neuropsychiatric symptoms. Mutation analysis revealed homozygous c.394C>T (p.R132X) mutation in the MMACHC gene. Serial magnetic resonance imaging (MRI) before and after the treatment are provided. MRI of the brain before treatment showed bilateral patchy focal hyperintensities in the white matter and cortical atrophy. After treatment with intramuscular hydroxycobalamin, oral folinic acid, oral betaine, normalization of MRI findings can be achieved in addition to clinical improvement. We present this case to draw attention to the reversibility of clinical and MRI findings in the late onset Cbl C disease after treatment.